As David says, it is likely that the carb slide is sealing well and when you turn over the engine slowly, enough air is dragged through to fill the cylinder and create compression.
I have recently helped a friend with his Comet rebuild and he had the same problem. Opening the throttle while kickstarting restored the compression. However the bike would only start with the throttle completely closed, so the drill was to turn over the engine with the throttle open a couple of times, bring it up to compression, pull in the decompressor and then kick with the throttle closed to guarantee a first time start. This only works if the pilot setting on the carb has been set correctly.
Forget about sticking vales etc. If that was the case you would get even less compression when turning the engine over slowly. The same applies to vales not sealing or cylinder bore leakage. Look for simple causes before exploring the difficult ones.
Regards Richard.