There are several reasons for the failure of combustion to occur, but one thing an operator can check for quickly is proper driver return. If the driver piston did not return all the way to the top on the previous cycle, then the combustion chamber at the top is in effect, too large; and the fuel cell cannot fill the chamber. If this happens, you may smell fuel because the fan will exhaust the unspent gas. Remove the fuel cell, battery, and any nails remaining in the magazine. Insert a long thin Allen wrench or thin screw driver into the nose area where the nails are driven from and push the driver up as far as it will go. Insert the battery and fuel cell and try again. If it still does not fire, it should be examined by a technician. If it does fire but continues to fail intermittently for the same reason, it most likely needs to be cleaned. A cleaning and maintenance guide was included in the tools carry case. Any further service should be completed by a qualified technician. Find your nearest Authorized Service Center in the support section on this web site.