You're probably right. The trap was left out several days and nights with cat food in it, and caught nothing. Then I put a sardine in it for bait, no newspaper or cardboard, and caught one cat, but not the particular one I wanted to catch. So we left it baited with sardines several more days with not luck. Then I put the cardboard in the bottom and baited it with a little canned tuna and promptly caught the one I was after. Did the cardboard help? I don't know, but I do know it sure didn't hurt.
Incidentally,
here is the Internet story I was talking about; third paragraph under "Setting the Trap".