Skyrim- Conjuration troubles?

I recently purchased the Conjure Flame Atonarch (or w/e) spell and the first time I tried casting it worked just fine, my familiar floated around the battlefield and helped me fight.

However, now I just cannot seem to get it to cast. I hold down the cast button and the conjuration animation begins, and my magicka even goes down to where it should be after the cast is complete. But as soon as I let go of the button to complete the cast my character just stops, no familiar in sight, and I get all my magicka back.

This is probably a very stupid question, but what is going on and how exactly do u conjure familiars?