For some reason, my alchemy skill stopped going up for making potions. I recently got the Thief Stone activated, so I am guessing it could be this, but other than that I do not know why it would not go up.

EDIT: I was wrong. It did go up, however it went up very slowly. When I had the Warrior Stone active, it went up fairly fast, but with the Thief Stone it seems to go up really slowly. Could the Thief Stone be glitched?

EDIT2: Problem was solved thanks to help

its because you get very low exp if you just make potions you know will work. the skill goes up much faster when you discover new properties for ingredients. if you experiment and try mixing new things, as you discover your skill will go up much faster.

Also, the more expensive the potion/poison, the more your skill goes up by making it.

Ah that makes sense now. I was making some very weak poisons. Thanks for the replies

If you are feeling lazy you can use these trainers for alchemy:

Journeyman Trainer
Lami in Morthal.

Expert Trainer
Arcadia in Whiterun.

Master Trainer
Babette in The Dark Brotherhood Sanctuary.

And if you are tight on the coin, just pickpocket the money back.
Train once, steal etc. Do it for the five times. You will probably level, so do it all again.