Skyrim 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.

Thrasher said:

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

Click to expand...

ahhh that makes sense. Thanks for the clarification!