» Sun May 20, 2012 5:34 pm
It's a known bug. I haven't checked into it recently (last two weeks?) on UESP, but as others have said, there's some conflict between their standard gear vs. better gear when it comes to bows and arrows. That is, if they have any arrows in their inventory (other than the hidden/standard arrows they're issued), they'll revert to their original bow. Sometimes this will happen even if you don't give them "non-standard" arrows because the arrows will enter their inventory via the game environment (e.g., they get shot w/ them or pick them up off the ground).
The best you can do, I think, in response to this bug is that if you notice that they've reverted back to their standard bow, check their inventory to see if any of those "strays" remain (if so, remove them), take away the "good bow", then give that bow back to them (to reset whatever compare function the AI is supposed to utilize in determining which gear to use). Keep in mind that if they've already got a melee weapon equipped, they won't necessarily switch to the good bow immediately, but should use it the next time they engage in archery.
The last I heard, it was supposed that this bug may only be relevant for those followers whose standard gear includes a bow and 15(?) arrows.
BTW, this bug can also be exploited in your favor for a reliable supply of arrows. If you equip followers who exhibit this behavior with two Elven arrows, e.g., then, yes, they'll use their old bow; however, they'll also have an unlimited supply of Elven (Ebony, whatever) arrows. This of course means that you can recover all the "spent" elven arrows that they leave in the game environment.