Know the feeling.
My latest vee wouldn't return oil to tank. Checked pipes, took timing cover off, changed oil pump for a NOS (it's Triumph) etc. No flow. Checked feed from sump, timing case off again (at least 3 or 4 times) and couldn't see anything wrong. After last strip (didn't do anything, just re-assembled in despair) it now pumps like a fire engine. At last.