First decide whether the cistern is flushing at all
A weak flush and no flush are different problems. If the handle or button moves but nothing happens, the link between the control and flush valve may have failed. If the cistern empties but the bowl does not clear, the water volume may be too low or the toilet may be partly blocked. Listen for the cistern refilling afterwards. A toilet that never refills has an inlet-side fault, while a full cistern that will not release water points back towards the flush mechanism.
Buttons and handles fail in different ways
Traditional lever cisterns use a mechanical link that can disconnect or wear. Push-button toilets often use cables, rods or pneumatic controls depending on the design. A stiff button can also stop the valve lifting far enough. Concealed cisterns hide most of the mechanism behind a flush plate, so forcing the control is a poor idea. If the button has suddenly become loose or dropped into the panel, the problem may be accessible without removing the toilet, but the correct access route depends on the system.
A weak flush may be about water level
Look at the cistern water level if you can access it easily. If it is well below the intended line, the fill valve may be shutting too early or the supply may be restricted. The toilet can then flush, but without enough water to clear the bowl properly. Scale around rim holes can also reduce the way water enters the pan. If the cistern appears to operate normally but the bowl rises unusually high before draining, a partial blockage in the toilet or soil connection is more likely.
Do not keep flushing a possibly blocked toilet
Repeated flushing can turn a partial blockage into an overflowing bathroom. If the bowl level rises close to the rim, stop and let it fall before doing anything else. A plunger designed for toilets can clear soft obstructions, but avoid pushing unsuitable objects deeper into the trap. Wipes, sanitary products and other items do not break down like toilet paper. If the blockage returns quickly after clearing, something may remain in the trap or further along the drain.
What a plumber will check
The plumber will separate the cistern mechanism from the drainage side. A broken flush valve, cable or inlet valve can often be repaired without disturbing the pan. If the flushing action is healthy but waste is not clearing, the toilet or drain may need mechanical clearing. Concealed systems can add time because replacement parts must match the frame or valve body. A good diagnosis comes from watching a full flush cycle rather than replacing whichever cistern part looks easiest to remove.
How to describe the fault
Tell the plumber what the button or handle feels like, whether the cistern fills and what happens to the water level in the bowl. Say if the problem appeared suddenly after something unusual was flushed. A short video of one attempted flush can communicate more than a long written explanation. If the toilet is wall-hung or has a concealed cistern, include a photograph of the flush plate because the brand and style can help identify the internal mechanism before the visit.
A replacement mechanism should suit the cistern
Modern flush valves are not all interchangeable, particularly in compact and concealed toilets. If a replacement is needed, ask whether the new mechanism can be serviced through the existing access opening in future. A universal part that only just fits may make the next repair unnecessarily difficult. Where the toilet also fills slowly, mention that before the plumber changes the flush valve because there may be a separate inlet problem. After the work, check both a normal flush and the cistern refill. The bowl should clear without the water rising unusually high, and the cistern should stop filling cleanly. If the flushing action is still poor after the mechanism has been repaired, the pan or drainage side deserves further investigation rather than repeated adjustment of the cistern.
This page is general guidance about toilet wont flush; the cause in an individual home still needs to be established from the actual installation.