I know a little bit about it.

My sis worked in the federal government division to check Medicaid (not Medicare) fraud. First, they were under powered. Only have less than 100 people for the whole country. Second, federal government funds the Medicaid program, but States administrate it. So their division can only assist states to investigate Medicaid fraud, have no authority to investigate directly. But many states have no manpower or interest to do it, because they got the money from federal government.