Jacob - 100% correct.
Little update to the mousetrap project. After finding bare yellow rods for a few days (meaning something was being eaten but not captured) we surmised that the remaining rodents were too small/light to tilt the mechanism, versus the inertia of the counterweight. So I removed the counterweight, and caught three more.
Its been 48 hours and no more mice. Granted the weather is changing so they might have left the premises, which is also fine with me.