Tubulars can be prone to valve/tube leaks. Most have the valve held to the tube via a flanged base and compression nut. Sometimes people who go crazy tightening the PV valve nut end up turning the valve slightly and loosening the flange nut inside the tire. You can test for this, or even fix it, by pinching the tire against the valve nut, and turning the valve to counter clockwise. If it moves at all that was the problem.