I take it that your using a single gauge tester, try rocking the pistosn 2 to 4 degrees either side of TDC (carefully )with pressure and see if that helps Then add several squirts of oil into the cylinders to see if that helps, if it does I would replace the rings remember that the rings seal against the cylinder walls and the piston ring lands, both do wear Can you hear any leakage out of the valves?