When I flush the toilet, I have to hold the handle down for about 10 seconds to allow the water to get to the bowl before the flapper closes. The drain holes are clear and the flush works, but I shouldn't have to hold the handle down. The house is 42 years old, but I know the toilet parts have been replaced at some point. Any suggestions for how to fix the flapper so it stays open when it should and closes when it should? Thanks!