Throw away the plunger and get yourself a ridgid closet auger which goes for about $30. If there is a clog on the trap way the auger will push out or retrive it. If you still cant clear it with the auger, you might have to remove it and see if there is any obstruction on the flange or bottom outlet of the toilet. I have often found that wax rings with a horn cause a lot of back ups. So before you do anything get your self an auger.