Hi!
It could be a worn cylinder /piston!
Have you taken the engine apart and looked at how the piston to cylinder fit is? Any carbon build up?
How is the piston to cylinder fit? If you try to push the piston through the cylinder, does it go straight through?The piston should never go trough completely with just pushing it with your fingers, if it does...the piston is worn out! The piston should stop a few millimeters from the upper edge of the cylinder if all is well.