With nearly all two strokes, the air and fuel mixture is sucked in through the crankcase so you have "crankcase compression" - you should feel resistance to pulling the starter twice per revolution, compared with once every second revolution for a 4 stroke. If the crankshaft-to-crankcase seals leak, or the piston doesn't fit the bore well (scuffing, striations etc.) the crankcase compression will be very weak even if the piston rings are in good condition.
I suggest you check whether you have a pronounced crankcase compression as well as a cylinder compression each revolution. You should also measure the piston ring gap if you haven't done it.
There are many things other than loss of compression that could be causing the symptoms you have described.
