Probably the most-asked plumbing question. The flapper not closing on or establishing a suitable seal around the flush valve is the most common problem. If the flapper isn’t closing, jiggle the handle. This should usually solve the problem. You will need to get the flapper replaced if it is worn out and not properly sealing. If the flapper does not close properly, water will seep into the toilet, and the tank will not fill with enough water to push the float up and switch off the water supply.