I crafted a legendary glass bow and enchanted it for my companion Aela, but when she enters ranged combat she still reverts to her default hunting bow. Are there certain weapons followers cant use?

Some followers won't switch bows. Call it a bug. I think Mjoll is one of the few who will switch. edit: Wait. Don't listen to me. She's got the same issue.

Yes, it's bug. You have to take the default items from them for them to use your gifts, sometimes. There's a console command for that. But you'll have to use the forum search function to find it.

I was aware of the removal of the default weaps but it doesn't even show a hunting bow in her inv, it just kinda spawns a new one lol :s

This is an aggravating series of bugs, caused initially, I think, by the developers not wanting to leave a follower naked and weaponless if they are dismissed.

Essentially each follower has a default set of kit that does not show up in the standard inventory that the player can see through the game. So through the game interface the player cannot remove any of the default items.

If the player gives the follower kit which according to some, unpublished, algorithm is judged better than the default item of the same class then the follower will use that item. The figure used to decide which item is better seems to be the base damage caused/resisted regardless of any enchantments. I also suspect that the followers own skill levels are ignored, at any rate I have had Lydia, who has much better skills in one handed than two, insist on using a battle axe rather than a very good sword, and Aela put on a suit of steel armour even though her light armour skill is more than twice that of heavy armour.

Anyway, however that algorithm works it seems that it doesn't work with bows - well most of the time it don't - and followers will continue to use their default bow despite having one with much higher base damage in their inventory. If you are on a PC there is a way around this - remove the default bow through the console. Before setting out how to do this there is one other, related, bug that I should explain.

If you dismiss a follower they will revert to their default kit if you have given them something else. However, what appears to happen is that the game creates a copy of the kit, certainly it does the weapons but I am not so sure about the armour. So, for example, Aela has a hunting bow as a default weapon, I take her on as a follower, give her some different gear and subsequently dismiss her. If, through the console, I check her inventory now I will see she has two hunting bows. I also suspect the same thing happens if I simply give her a new bow and then take it away again forcing her to revert to her original. So it is entirely possible that a follower will have multiple instances of the same item in their invisible inventory; I have read of one case where a player found that Lydia was carrying around 15 of her default bows. As an aside, though the items are invisible they do count against weight allowance, so that a follower's carrying capacity is gradually reduced.

To remove extra items from a follower you'll need the item ID. This can be found through the ShowInventory console command, though that has its own problems which I won't go into here, or simply use Google. For the moment let's just use the hunting bow, as carried by Aela, as an example. The ID for this bow is 0010e2dd

Save the game (just in case), get Aela in front of you and open the console.

Type: GetItemCount 00102edd
Click on Aela on the screen and press return

This will give you the number of hunting bows Aela is carrying in her invisible inventory. Now we need to get rid of them all

Type: RemoveItem 00102edd #
(where # is the number of hunting bows she is carrying).

Exit the console.

If you haven't already, give Lydia the bow you want her to use and this will be what she uses from now on.

Note Well: Hacking around with followers default items can have consequences down the line. I removed Aela's default Ancient Nord Armour and for a while she was wandering around in a full length frock. Playing with the console can bugger up your game.

P.S. One thing I haven't had time to systematically try is to give a follower such a Aela an uprated light bow (e.g. Elven).

Edited for typos

basically any companion with a hunting bow has this bug where they always seem to prefer it in combat (they'll equip the bow you give them out of combat) and no matter what you do to remove it from their inventory (even console commands) will not work, they will just spawn a new one.

I gave Aela an enchanted glass bow too... then I tried taking her hunting bow out of her inventory, chucked it onto the ground and she was using a hunting bow that wasn't there.

Eventually she will spawn a new hunting bow, but you can repeat the console command when that happens.

All followers (except Cicero and the Dark Brotherhood initiates) gain an unplayable hunting bow and twelve unplayable arrows when they join you. It is not removed when they leave your service, and another will be added if they follow again. They seem to prefer it as a ranged weapon because it cannot be disarmed. On the plus side, when they fire from it, they do not use up ammo.

Lydia will not switch shield. And yes it leaves Aela pretty useless in ranged.
TCG.esp solves the problem well, also give you the option to put housecarls in something more comfortable at home.

i have removed the bow from Lydia and she just kept using the invisible default one.

the same would be true for Aela especially cuz she is a ranged character.

I do it slightly differently, and avoid the "duplicate" problem.

I target Aela in the console, enter removeallitems player, which gives me all she is carrying, including the hidden stuff. Now she's in yet another default kit. So I then enter removeallitems. Poof! She's in her birthday suit, which the player can't equip.

I then give her what I want her to have, starting with the bow. The moment a weapon or bit of armor is traded, her default kit reappears. You just can't avoid it. However, she will equip what you give her. IIRC, followers only benefit from certain kinds of enchanted gear. I think it's the base+percentage type, and in any event, they ignore enchantments when choosing what to equip.

Oddly enough, this occurs to the player, as well. Let's say you have two axes, identical except one is enchanted or improved. If you equip one and attempt to store it on a weapon plaque, both axes will be applied, with one falling to the floor (or through it).