My Valve Spring Compressor - Heads On/In Car

I forgot about using rope to hold valve closed.
A very very long time ago I had to put those umbrella seals on the guides and so I hooked up the compressor to each cylinder and had the tool that bolted on each rocker stud (GM) it wouldn't work, kept opening valve, letting air escape......Guy told me to tap each valve and than try it, worked like a charm