Could mice be escaping traps because the traps are being reused?
It's unlikely to be a major issue with reused traps. Just check that there's not a little rust or something developed that is causing them to stick.

You could try a square of chocolate as the bait. If you heat the pin on the trigger platform, you can push the chocolate on and it's stuck there. That makes it harder for the mice to get away with the bait.