Compare the water level in your tank to the fill valve. Check the model number on your fill valve to identify which one you have. To drain the tank, turn off the water supply and flush it. Older valves often need screw drives to modify; however, modern valves can be automatically updated. Try cleaning your fill valve if the water level isn't the problem. Your toilet would often operate badly due to obstructions or debris accumulation, and removing any clogs by running the water with the top off may cure your issue.