I assume you mean that the starter always turns the engine over, but the engine will not fire. If that is the case, I would try two things:
First of all, try using the following starting technique: Turn the key to the position where the warning lights on the dashboard come on; and wait with the key in that position for a full ten seconds. Then turn the key the rest of the way to start the engine.
The reason this works is that the electric fuel pump in late model fuel injected vehicles has been moved to the back of the car, inside the fuel tank. That leaves a long fuel line between the pump and the engine. It takes about ten seconds for the pump to refill that line; and build up enough pressure to fully prime the injection system. If you try to start the engine before the system is fully primed, it will be starved for fuel; and will be hard to start, or will stall after starting. When the vehicle sits for more than 10 minutes after it was last run, it cools down enough that the computer richens up the fuel mixture; which then makes it easier to start. But the first 5 to 10 minutes after shutting the engine down are always the hardest time to restart.
The other thing that makes the engine hard to restart is when the spark plugs have become worn or the plug gap has become too wide; or the plug wires have developed excessive resistance, or if the distributor cap and rotor have become dirty or carbonized on the inside or outside.