Is that just me, or is the spell learning chance on normal level inferior to what it should be?
EG, Edwin gulps a genius potion. He should have something like 95% learning spells, but when I try, he fails on 20% of the spells.
Because of this, I always play on the lower level of difficulty.

Your percentage is just a base chance. If you are attempting to learn a high level spell, there is a modifier penalty. It is equal to 10% per level beyond what you can currently cast.

Example: Edwin has a base 85% chance to learn a spell, and is 11th level, meaning he can cast level 6 spells. If he attempts to scribe an 8th level spell into his spellbook, his success rate is 85% - 20% = 65%. Note that this doesn't work in reverse. He has the same 85% chance of learning a 5th level spell as he does a 1st level spell.

Thanks, I didn't know, I will try with lower level spells.

There was a discussion about this in detail on the Interplay boards. Even figuring the penalties for learning spells beyond your level, the consesnus opinion was that the RNG is screwy and you fail more often than you should. One of the guys ran some tests that basically supported the consensus.

Yes, the failure rate is far too high in every BG game I have played. However, just gulp enough potions to get up to 24 or 25 INT and you should never fail. For some reason, the -%10 modifier doesn't seem to apply at that range.

Or move the difficulty below core, where spell learning never fails.

I've noticed the same thing. The SoA documentation does not mention it, but in the normal AD&D game, specialists (like Edwin and Jan) have a -15% chance to learn spells outside their speciality school. It may be that this was implemented but they forgot to document it.