Check the water supply valve behind the toilet and make sure it is fully open
Lift the tank lid and confirm the tank has enough water
Inspect the flapper to make sure it opens and closes properly
Reconnect or adjust the chain if it is too loose or too tight
Clear any clog in the bowl with a plunger
Use a toilet auger if the clog is deeper in the drain
Check the fill valve and float to make sure the tank refills correctly
Inspect the flush handle and lift arm for looseness or breakage
Remove any debris blocking the rim jets or siphon jets
Test for a blocked vent pipe if the toilet still flushes poorly
Check the wax ring and drain connection for leaks or shifting
Replace worn or broken tank parts if needed
Call a plumber if the toilet still will not flush