Zed101

Just as a routine check, try putting a little oil in the sparkplut hole, turn the enging over a few times. Recheck the plug for depth! It may be the wrong type and goes in too far allowing the piston to hit and not complete its cycle.
Just for a test, if you have a shorter plug, wheather it is the correct one or not, try it. If it allows the engine to turn over , Thats the problem!
The oil may help the system anyway.

Good luck,