I had to resort to not using Conjuration now as i only have a 20% chance of my spell actually working... It can sometimes take a few mins before my Atronach/Dremora appears...
It's caused by having Spell Absorb, having the Atronach stone + Atronach ability makes it nearly impossible to summon anything, it has also effected some shouts i believe, one of them being Call of Valor, making it useless.

http://www.youtube.com/watch?v=vCQ0XOYBxdE