You may have a flow pressure problem, that needs to be corrected. I would test the flow of your water system. Fill valve needs around 9 psi to opperate. If it doesn't get the pressure because other things are not working. Then you may have to install a ball type fill valve. These are old style lift rod brass fill valves. They opperate even when the pressure is at 1 psi. Though very slowly. If it was me, I would shut the water off to the toilet, remove the cap to the fill valve, and turn on another faucet, then slowly turn on the toilet and see what kind of a stream of water you get coming out. If your carefull you won't get it all over the house. If you see the streem is fairly good. Replace the fill valve with an old style. A little more expensive, but worth it. However if you find your pressure is bad. Its probably time to replace the pipes in your house with copper or PEX.

Raymond VinZant Plumbing Prof.