There are two main reasons why the reserve toilet tank will not fill with water: either a broken or worn toilet flap or a broken fill valve. Another simple reason that is often overlooked is that the water supply to the toilet has been turned off.
When a toilet is flushed, the chain from the handle lifts the stopper, also called the flapper valve. The toilet bowl fills as water flows in from the flush valve opening. After the reserve tank is empty, the flapper valve closes and the tank refills. If the tank does not fill properly, there are a few easy fixes a homeowner can try.
An obvious but often overlooked cause is water supply. A homeowner should first check the toilet's water supply valve and make sure it is completely open. Another check to make is on the water supply valves to ensure they are tight and not leaking. If the water supply seems to be fine, the problem might lie with a broken fill valve. If the fill valve has a build-up of grime, that may cause it to work inefficiently. To combat this issue, the homeowner can take the valve apart for cleaning and reassemble to see if that fixes the issue. A broken or worn toilet flap does not seal the reserve tank completely, allowing it to leak and the toilet to run constantly. Replacing the toilet flap will fix this issue.