A toilet that won't flush properly has a handful of common, easy-to-check causes. Work through them before assuming the worst.
Check the water level and flapper
Lift the tank lid. If the water level is low, the fill valve or float needs adjusting so the tank fills to about an inch below the overflow tube — too little water means a weak flush. If the flapper closes too soon, not enough water reaches the bowl.
Check the chain and handle
A chain that's too long won't lift the flapper fully, giving a weak or partial flush; too short and the flapper won't seal. Adjust it for a little slack. A loose or broken handle linkage also stops a proper flush.
Clear clogs and jets
If the bowl drains slowly or backs up, it's a clog — plunge it or use an auger. Over time, mineral deposits clog the small rinse holes under the bowl rim and the siphon jet, weakening the flush; clearing them with a descaler restores flushing power.