Pat,
When you say "blew" off, do you mean that you saw it blow off the tube nipple with some forceful pressure or that you just discovered it was off? If the latter, perhaps you jostled or bumped it while working on the gearbox. If you saw it blow off (or even if you didn't see it), do a check for blockage in the tube, in the rocker-feed banjo and, as Dutch mentions, at the side holes in the banjo bolts.
Richard L.