In order to check the lifters, you really need to start and run the engine with that valve cover off. It will run on the opposing bank just fine. After all, that's basically what AFM does, just using different cylinders. Anyway, if you have a bad lifter, it can cause the rocker to move when turning the motor by hand, and when cranking, but then when the engine actually starts, the lifter causes the rocker to stop moving. I know, because I had a bad lifter, and the compression test came back good. Then I cranked it over with the valve cover off, and everything looked fine and the rockers were moving. Then i started the engine, and bingo bongo - no rocker movement on #7 intake. Then I shut it off, and cranked the motor over without starting it - and the rocker started moving again.
When driving the rig, you could feel the missing cylinder kick in here and there and then cut out again.