Bill from Qu'appelle Valley in Saskatchewan is trying to figure out why his toilet tank just won't fill up. The flapper is in place but the water seems to be just flowing through.
Rubber flappers do get old and need replacing. If you use a chlorine puck in the tank to help keep the bowl clean, it will simply eat the rubber, shortening the life of the flapper even more. These cleaner chemicals appear good for the bowl, but wreck havoc inside the tank.
Just replace the flapper Bill. Make sure that there is nothing on the lip where the flapper seats down and that there are no nicks or cracks. Also make sure that the hinge and the chain allow the flapper to seat properly.
To test if the water is flowing through, either through the flapper or a leaking valve, see this test technique.