A weak or incomplete flush is often caused by a tank that is not filling with enough water. Lift the tank lid and check that the water level sits close to the fill line marked inside the tank (usually about an inch below the top of the overflow tube). If it is noticeably low, the fill valve may need adjustment.
If the chain connecting the flush handle to the flapper is too long or has come loose, the flapper may only lift partway when you flush, releasing less water than needed for a full flush. A chain that is too tight can cause the opposite problem, holding the flapper slightly open. Both are quick, low-cost checks.
If water rises when you flush but does not drain, or drains very slowly, the issue is more likely a clog in the toilet trap or the line beyond it rather than the flush mechanism itself. A plunger is usually the first thing worth trying in this case.
Mineral buildup in the small holes under the bowl rim (called siphon jets) can reduce flush pressure over time, especially in hard-water areas. A worn-out flush valve or a toilet that was never sized correctly for its drain line are less common but possible causes as well.
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