Somehow, they can tell when you're "gaming the system" by giving the wrong dishes to the wrong cat, or making false protests when they're "stealing" from the dish you really want them to eat from.
It's especially tricky when medications are being given or when there's a special food needed. No matter what measures you take, it seems, you're doomed to failure.
One cat needs a special food or medication, so you try to feed them while keeping the other cats away. "No way," says the cat, "I'm not eating. The other cats aren't here, that's weird, you're up to something, so I'm not eating."
It seems to help, in my experience, if you've made them "earn their meal." They're more willing to eat out of their own dish, and defend it to some degree rather than go steal from someone else (at least right away.) We have had two cats that we've trained to sit up for their dish. They'll at least eat the top off their own food before going elsewhere. The problem is, that once you start making them sit up for their dish, you've got to keep making them do it. If they learn that they don't have to, then they won't.
Another approach is to set down the food, and bide your time.
I manage to get the right drugs in the right cat by setting down the dishes, and waiting. Each will eat some of their own, then wander away from the dishes. Apparently paying no attention to the food at all. I'm doing a bit of moving around myself, also paying no attention to the cat food. Then, casually, almost as an afterthought, I'll
After a few moments of "distraction", I can be sure that when I turn around the drops will be gone, along with some of the "forbidden" food, with perhaps the sight of a completely innocent paw leaving around the corner.