Make sure all of your legs have a white milky straw around each of them (if this is a new-ish unit they legs may be wrapped in black heat shrink).
If re-seating the tube and the legs are all insulated, then there could still be an issue with the tube. If you have another AlienBee or XSeries light, you can try swapping the tubes. Also, make sure notheing is plugged into the RJ11 telephone port when doing this evaluation. Otherwise, it sounds like there is an issue in the head that would require repair.
To get the head repaired, please visit this link for full details.
http://www.paulcbuff.com/repair.php