[Preegovesem]

If she's an adult, she should be heavy enough. Did you line the bottom with newspaper to hide the trip plate? She may just be putting her front paws right over it. Use a very light layer of paper--just enough to obscure the bottom without interfering with the trip mechanism. In my experience, either no paper or too much paper leads to cats eating the food out of the trap without setting it off.

If that doesn't work, you could try to find someone with a drop trap, which are very effective. Look it up on youtube and you can see crazy catlady demonstrations