WHAT THE!?!

Basically, i wanted to cast a night eye spell that needed illusin level of 25. I got it to 20, then found a trainer and got them to train it to level 25 as i was too lazy to. When it got to lvl 25, it still said i could not cast the spell. I have enough magicka, and it still says 'skill level too low', i got my level to 30, and it still says the same thing. I only need the spell to make a night eye helmet for help on the stealth quests, so it is not vital and illusion is not one of my major skills, so it is not exactly vital, but i want to know, WHAT HAS HAPPENED!?!

Please help guys

Small glitch........ or it could be that you have a skill enhancing item equipped. Normally you should be able to use the spell but there are the rare cases you can't(you just happened to be the unlucky one this time). Go into your stats and look at your illusion. If it is highlighted green then you have an item that makes it seems stronger then it really is. Just know that those items can be good, but at times a very big hassle.

fablelord's right. Also, and I have run into this quite often, if you have a spell that won't work even though your skill is up to where it needs to be, just level up. Sometimes I think the game won't really recognize an increase until the next level. Especially when it comes to "side skills" (e.g., anything other than restoration, destruction, etc.)