Problems like this have come up here several times in the past, and there's never been a "one-thing-fixes-all" solution for it. Outside of cleaning the cart slot and the leads on the carts, also make sure that none of the pins in the cart slot are bent (even a very slight bend can lead to problems), and that there's nothing that may have fallen in there that may prevent a perfect fit for the carts (although this doesn't seem to be the case for you). Outside of that, there's been some carts that I could never get to work properly no matter what I tried, and that was usually evidenced by the cart having a different fit in the slot than most others. Again, though, with this happening with most of your carts, I doubt it's the issue with all of them. Would be interested in hearing if you do end up figuring out a permanent solution.