First of all make sure you have the throttle opened up - sometimes even 10% is enough. If that is not the problem the try this:
Open the aircraft config file and find this entry -
[piston_engine]
power_scalar =1.00000
Change the power scalar to 1.10000
That usually lets the engine start. You now will have an engine power increase as well, so the next step is to start lowering the scalar number to the lowest point where the engine will start so you effect the engine power as little as possible. Try 1.07000, then 1.05000 etc.
If the config file does not have this entry, simply copy the above and paste it into the file. Doesn't really matter where in the file as long as it is separate from the other sections.