All depends on the model of the flush valve. Most though have a "diaphram"
which is easily replaced. Another possiblity although not common is the water
supply line. It could have been tightened too tight at the valve end. This end
has a ferrel that squezzes into the hose when the nut is turned. Turned too
tight and the ferrel pinches the pipe, leaving it vulverable to clogs.