I know it sounds trivial, but from what you wrote, it could also be that you ran out of memorized spells. A little more info on how to reproduce this possible bug?
I had full spells i cast armor..then i cast 1 magic missile and tried to cast another and it never did....it did this for awhile even restarting the game then it just started working..maybe i wasnt letting the cooldown get done??
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A little more info on how to reproduce this possible bug?
Have you memorized it?