Check the toilet tank water level and raise it to the marked fill line
Adjust the fill valve float upward
Turn the fill valve adjustment screw clockwise if present
Replace a faulty fill valve if the tank will not fill properly
Make sure the flapper seals correctly and is not leaking
Remove any partial clog from the toilet trap or drain
Clean the rim jets and siphon jet in the bowl
Verify the refill tube is inserted into the overflow tube
Raise the water level in the tank slightly and flush again
Inspect the water supply shutoff valve and open it fully
Replace a worn flush valve or fill valve assembly if needed