Check the toilet tank water level
Adjust the fill valve to raise the tank water level
Turn the fill valve adjustment screw clockwise if present
Raise the float on older ballcock-style fill valves
Make sure the shutoff valve is fully open
Inspect the flapper for proper sealing
Clean the fill valve and inlet screen
Replace a worn fill valve if needed
Check the refill tube placement in the overflow tube
Ensure the refill tube is not kinked or blocked
Remove mineral buildup from the rim jets
Clear clogs in the siphon jet or trapway
Replace a faulty flush valve or flapper chain
Verify the toilet is not leaking into the bowl
Call a plumber if the water level still stays low