You have to have a really good battery to not only crank the engine, but if voltage drops too low while starter motor is engaged, the ignition system will not have enough power to fire. If you have a volt meter, put he leads on the battery, then hit the starter. If voltage drops below 11.5 volts, that may be your problem. Also run a battery that is rated for your jet ski. I have seen people try to run a cheaper battery with lower cranking amps, they will wind up with this problem. Always run a battery specified for your craft.