Open the tank and watch the flapper...how far up is it being pulled by the handle? If there is very little distance between the flapper and where the water leaves the tank to flush the bowl....the pressure of the escaping water can close it. To fix, move the "chain" between the handle and the flapper to make the chain shorter, thus pulling the flapper farther away from the exit. Hope this helps. If it doesn't, try replacing the flapper with a new one. Thanks, Janes!
