Peanut butter works best because it sticks and is not a solid piece, so they have to work at it. Solid stuff they can take without tripping the trap.
Though, I could glue a piece of dog food to the trigger.
Also I just had one set trip, I'm thinking something slightly shifted and the resistance might be higher than the pull down resistor's resistance. I have a 10k resistor for that particular trap. The other has a 10k and one has a 1k. Could not find another 10k. I will need to take some stuff apart to see if I can come across something like 20k or even higher.