Are you sure the glowplug is fully tight and seated?

You can check the head for leakages. Get a short glass, fill it up with water almost to the brim, then put the head of the engine in the water...almost up to the exhaust port...and turn the crank slowly. If you see air bubbles being produced, that is your compression.

You can tell is the piston is cracked by looking at it with a magnifying glass, while heating it up slowly with a heat gun, or torch or something.

You should be able to tell by looking at the piston if your leaking past the ring...there will be "blowby" You'll know it when you see it.