You have to just keep trying. You might try putting the food in the trap daily so she gets use to going in and out to eat, without setting it to trip. Then when it appears she is use to this, set it. Here's a site showing the best type of traps. http://www.animal-care.com/cata06.htm

Also check out this site for pointers on trapping:

http://www.alleycat.org/

Type trapping in the search box and it will bring up some guidelines on trapping.

Good Luck!