Be patient with the trap. Once you find the honey-hole, so to speak, you should put a big dent in the population.

What are you baiting with?

Also, you can throw some hotdog pieces around the trap to help sucker them in.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."