Check that there is actually a spark between the two electrodes of the plug when pulling the start cable by removing the plug and holding the side of the plug body to a metal part of the mower using a pair of pliers with rubber covered handles. If you pull the cord and sparks arc bewteen the electrodes then it must be a fuel problem - or you may have 'flooded' the spark plug with fuel by continually trying to start it. When you take the plug out if the plug is wet with fuel dry it off, check for the spark as above and try again. Good luck.
Paul
April 2008