I tried using an Armorer potion for the first time yesterday. It said that weapon and armor improvements would increase by 30%. So I try to improve a pair of eleven boots without the potion first. I can improve them from 20 to 32 (exquisite). Then I reload, drink the potion, and try to I improve them again. I can still only improve them to 32. Shouldn't it improve to 35 (20+12*1.3)?
I later try the same thing with a potion of enchanting and get the same result (i.e. my enchantments are not any stronger).
Am I doing something wrong?

There are "level" or "steps" to the smithing process. If the percentage didn't boost you to the next step of expertise it will have no effect.

And potions can't make up for a lack of smithing ability- what was your smithing level? If potions could do that, no one would master smithing.

You didn't run out of the required materials to improve the item, did you?

See here for detailed info:
http://www.uesp.net/wiki/Skyrim:Smithing#Item_Quality

Essentially, the potion increases your smithing skill by a %; it does NOT effect the % of the improvement directly. Thus, the higher your smithing skill, and whether you're working with a perked item, contributes dramatically to the liklihood that the % boost will be helpful.

It is best to think of skill boost potions as a way to avoid grinding to the next level, as long as you are close enough to the next level for the potion to give you the next step.

Of course, stacking potions is another thing entirely......