It sounds like the wiring is not making a good connection, either through a broken wire or a bad connection in the endcaps. Sometimes, the bulb rotates out if you bump the reflector and does not make a good connection.
I thought this.. i checked it all.. it flashes for about a second.. but only two of the bulbs.. then stopped doing anything. the new ballast worked for a week but now does the same thing.. I thought wiring too. but want to be sure