We had that problem. Found out that that the sewer drains for us and our neighbors converge to a 6 inch drain that goes to the street. Called CDD utilities (their number can be found here,
Village Community Development Districts) and they sent out a vacuum truck. They told us that if the problem was our drain line we would have to pay a service charge, but the problem turned out to be rocks in the 6 inch pipe that was forming a dam that kept the line from draining into the street. They removed rocks and other construction debris and we haven't had problems since. A plumber will not check this out and that's why it returns for some. And the people who came out for us told us that this is a common problem.