You may have got dirt in the water that is blocking the jet.
depending what type of syphon you have depends on how to fix it! the common one is to isolate the water from your toilet and then undo the mains inlet and remove the actual syphon , you should come across a small piece of plastic with an hole in it (check it clean it replace it) then replace everything in reverse order not to loose any washers.
charlie s., November 2006
