Soul Gem question/bug?

I have empty soul gems of every type specifically a lot greater and grand. However seems about 50% of the monsters I kill with my soul trapping weapon I get the message "no large enough soul gem available" or something along those lines. What am I missing or is this a bug?

Thanks for any help!

They may be humans you are killing? Those need black soul gems.

Originally Posted by Thrasher
They may be humans you are killing? Those need black soul gems.

ahhh that makes sense. Thanks for the clarification!