I also use hav-a heart traps but I upgraded their bait holder by installing a nut & long bolt through the plate. This allows me to better hook the bait onto the trigger plate (like pushing a half apple into the bolt). The critter would then have to pull off the apple which always triggers the plate. To entice the critter to enter the trap I place little slivers of apple at each entrance. If my setup is hard to visualize and I can post pics later.

I also have a larger trap that local animal control uses to capture feral cats where the critter has to walk over the trigger plate to get to the bait. This one never fails.

Interesting. What I do is somewhat different. These are two-door traps, and I leave one door closed and latched. I put the bait just inside the latched door, so the beast has to walk over the trigger plate to get it. That's ALWAYS worked. Squirrels are a little reluctant to enter a trap with only one door, but I seem to get them anyway. Here's the reason I do that. In the small trap, the tail is someone long compared to the trap, so if their nose is over the trigger plate when the trap trips, their tail sticking out backward can prevent the door from latching. Then when it snaps shut without latching, they just back out. I need them to be completely inside the trap when they trigger it. But that's a good idea. I might try anchoring the bait on the trigger plate on the large trap, though.

Might not be a bad idea to rig a trail cam to cover the traps... hopefully that will at least identify the culprit.

Well, if I had a trail cam, that would be a nice idea. They are pricey. My alarm should allow me to quickly identify the culprit. But I'm hoping the alarm doesn't scare it off. I've ordered the alarm (should be here in a couple of days), and I've set up a spotlight shining on the trap that I can easily turn on from indoors so I can see it at night. I'd really like to watch and see exactly how this fellow is getting my bait without tripping the trap.

Annoyingly, the only vibration alarm you can get is one that sounds where it is mounted. This thing has a remote control, but it's just to turn it on and off. The alarm sounds outside, on the trap. An alarm that sounds inside, on the remote would be better. Of course, that means that if it turns on, I can quickly turn it off from inside.

There are some cheap remote doorbell alarms ($14 here) that are motion activated. My local "good stuff cheap" (Ollies?) store has them. You hear the alarm indoors, not the critters. Amazon has it for $20, we have one on our driveway entrance to give us a heads-up. It's sensitive enough to pick up a bird flying by it. Each end (transmitter and receiver) is battery operated.

Well, I got my "nudge" alarm. Pretty nice. It goes off if the thing is nudged or bumped. Smooth motion doesn't set it off. So someone bumps it, and it sounds. An unwanted feature is that it has a "mistake" option. That is, if you first bump it, it sounds briefly, which is supposed to warn you. But if you bump it a second time, the assumption is that there is intent to steal, and it sounds continuously. So my pest really ought to bump it TWICE. I have it balanced in a knock-it-over location in front of the bait.

So I've set it up for several nights in a row, and .... NOTHING. The critter stopped coming! Bait not getting stolen. After raiding my trap for ten days straight. Gosh, does it KNOW I now have an alarm? Will keep it up for a while, but this is continuing to be exasperating. I have the alarm turned off in the daytime, and the baited trap keeps catching squirrels.

I was thinking it might be a possum. They’re a menace in my veggie garden. They apparently like to eat everything.

I have lots of possums, and trap and relocate them every year. It seems that this one is smart enough to avoid being trapped while stealing bait.