The flushing mechanism as a whole is usually very dependable so should yours fail this should be put down to just plain bad luck and you can bet that it will pick the time you want it to flush it will fail.In the first instance, just try giving the cistern and flushing handle a really good visual inspection just to look for anything obvious. Ashford Plumbers can inspect plumbing and also diagnose and repair problems. If there is no positive result from this, it will be a failure of one or more of the internal components that make the flush unit.Note: The likely candidate is called a diaphragm washer which is fitted to the underneath of the siphon.Replacing a Defective Diaphragm Washer:1). Turn off the water feed to the cistern and empty it of all the water by flushing. Remove any remaining water from the cistern. Take the siphon from the body of the cistern (take photos t help as you continue the task), with the siphon removed you will now be able to see beneath its base and see where the perished diaphragm washer sits. Unhook the links to the lever arm, if the arm of the float operated valve gets in the way, you may need to remove this also. An Ashford Plumber can take on any plumbing task.2). Remove the hook from the shaft that pulls the diaphragm, it should allow the diaphragm housing to drop from the base of the siphon. Fit the new washer and fit the parts back in reverse order (check the photos you took). The joint washers must be in a good condition, if this is not so, use PTFE tape to wrap around the joining parts (not on the threads).Finally: Reinstate the water supply and check all around the WC for any leaks.