Fish is right - you can fine tune the whole arrangement by adding small weights to the see-saw.
The problem with the see-saw is that it acts as a lever, so the further past the pivot the quarry gets, the more downwards push it with generate.
So when the little mouse gets right to the very end he could be generating a relatively large amount of downward force. If you set the see-saw with too much counterweight them mousey may not tip the see-saw fully but stands a chance of getting the back end off the ground and buggering up the locking 'U'.
More importantly even a rat will gener