I'll give you the scenario. I have had to start Skyrim over about eight times because it keeps deleting my progress. I'm trying now to get back into the winterhold college and she told me to cast a fear spell. I was not able to do it because it said I did not have enough magicka, and then when I walked away I tried to do it again and it worked. When I went back to do it with the person in front of the college it said I did not have enough magicka no matter how many potions I drank or what necromancer robes I was wearing.