If the engine has been sitting for a long time since those "10 to 12" runs since overhaul, the ring could be stuck depending on what type fuel it was run on and how long it sat.

Even though the valves move, it doesn't mean they are sealing. some oil down the intake and exhaust holes with the piston on the compression stroke then turning it through will reveal lots of bubbles if the valves are not sealing.

Also check the head bolts for tightness.

My guess is the engine has a lot more than "10 to 12" runs since overhaul.