Can a plumber please tell me why my toilet is still leaking after we have changed the wax ring twice now?

The toilet started leaking at the base so I called a plumber to come and reset the toilet. It was ok for a couple days and then it started leaking again so I called them back out. He changed the wax ring again and it was ok again for a couple days and now it's leaking again. What do I do to stop the leaking?