I've experienced this issue multiple times lately but couldn't quite put my finger on it until last night:
A magicka skill wouldn't activate, even though I had a full magicka bar. It was as though I was trying to cast with no magicka available. I could use my weapon, but no matter what skill I was trying to use, it would not cast. It wasn't for lack of a target either, and wasn't lag either, I was trying to use heals. This most recent time I noticed my stamina bar flashing when trying to activate Polar Wind, a magicka skill (and there is no stam morph so that cant be it)!
Has anyone else noticed this before, or anything similar? I'm pretty sure it is stamina related because I was knocked down, used break free, and was left with an empty stam bar.
Was hoping to actually figure it out before submitting a bug report.