Board Thread:General Discussion/@comment-107.194.104.34-20130214005853/@comment-64.134.230.197-20130417044434

What type of soul gems do you have in your inventory, and what type of creature are you soul trapping?

If you have small soul gems ex. Petty, Lesser, Common, you can only fill those with small type of creatures.

If you have large soul gems ex. Greater, Grand, you can fill them with larger creature types.

If you have a black soul gem, you can soul trap NPCs.

Or maybe you know this already... then maybe the soul trap effect wore off before the soul could be trapped in a gem.

More info see: Soul Trap (Skyrim) and Soul Gem (Skyrim)