How can I grind sesame seeds in my coffee grinder better?
Grind the sesame seeds separately using a pestle and mortar (if the seeds really have to be ground).

Grinding sesame seeds manually isn't as hard as grinding the other nuts/seeds that you're mentioning - I think it's just the size that helps them not be hit by the grinder blades so effectively.

So, pestle them down and add to the mix afterwards.