You can't just put a trap in the middle of a room and expect the mice to be in the exact position when they are taking the bait.
Make it so that the mouse has to approach it in sush a way that it will take the bait, and set the trap off. Try boxing the trap in, so that it can only be approached from the front. Put it in the corner, with a brick alongside it and a piece of wood across the top, as if it was in a narrow tunnel.