I have a problem with my kitchen sink. When I use my disposal the sink backs up and water will not drain. I have had a plumber look at it, he snaked the line and found no blockages. He thought it was a venting problem, so 287.00 later he installed a vent pipe under the sink. This worked for 2 days, but today we used the disposal to get rid of some fish (no bones) and the sink backed up. The only way to get the water to drain is to plug the other side of the sink and run the disposal until the water starts to drain. No other drain in the house is affected. There is no food stuck in the disposal or trap when the water backs up. What could be causing this?