It’s possible that rats have been eating the bait. They make numerous trips, nibbling it until it falls off of the spike. Then, they either eat what’s left under the Timms, or take it away. I used to find nibbled apples in my traps all the time until I changed things.
A mistake that I’d been making for years was to use a piece of apple that was too big, usually half an apple on my Timms. These were usually too big for possums to get a good bite, so the apple would spin away. A rectangular piece that’s about 50mmx30mm has worked, and skewer the bait horizontally. Don’t put in the core of the apple, because the seeds prevent it from skewering well, and this prevents seeds being eaten. This technique makes an apple go a long distance, too, saving a bit of money over the long haul. It’s easier to dice an apply into bait when you use a really big apple.
Next, give the apple chunks a dusting of cinnamon, which rats dislike, but possums are really fond of. The cinnamon keeps birds, insects, and invertebrates away, because they can’t detect the sugar that the apples contain. I put pieces of apple in a sealed plastic container, sprinkle in some cinnamon, and give the container a good shake, which gives every surface a light coating.
The cinnamon seals in moisture that would be lost without a coating, so the apples stay firm longer. This is especially important during hot, dry weather, when apples can shrivel up in no time. I have trapped a few possums with a shrivelled-up apple, but it’s rare.
If this won’t work large-scale, and apples can’t be coated with cinnamon, you could try placing rat traps close to your Timms. You’ll get some rats before they raid your Timms, and ones that return for more, but it’s all gone. Just use a bit of apple in the traps. If you’re trapping feral cats, a site with a Timms and a rat trap might also be a good spot for kill traps (like a DOC 250) or a cage, because rat and possum carcasses can be used as bait. Checking 3 traps at one site will also be a time-save, because there will be fewer stops to make.
I hope this helps!