I had this problem too!! We got one of the electric litter boxes which helped a little until they learned to open the "treasure" box full of tasty morsels!! YUCK!!! But finally what worked for us was hot sauce directly on the poop and one taste of that did the trick!!!!! Sometimes the temptation is too great and they go back so we get out the hot sauce again and it fix's it again, you end having to do it every so often with some of the more stuborn ones but it's the only thing I found that works well.
Max Piper Emma