If you get a spell what if it doesn't appear on your powers?

I'm sorry, I do not understand your question as written. Please rephrase and use details. Also, provide a chapter number if possible.

Look in your spell book and switch it with a spell in use

Maybe it is a glitch. Close the game and rejoin it. Maybe it'll work. If not after at least 5 tries, it should be a bug. (but I highly doubt it is a glitch.)