worn cam lobes or collapsed lifters could cause the valves not to open up all the way..... but wouldn't cause bad compression, bent valves, could cause the valve not to open all the way but at the same time not seal well either.... try a cylinder leakdown test, get an air fitting that screws into your spark plug holes.... bring the cylinder you want to check to TDC, screw in fitting, hook up to compressor, if you hear air coming out the exhaust or intake and you are for sure at TDC, you probably have bent valves, if you hear air at your filler cap, its rings, if you get bubbles at the radiator cap, its a blown headgasket, or less likey, an eaten thru cylinder wall