I am replacing 2 ballasts in an old 4 tube fixture with a single ballast (as per our local elect. supply store). I have connected all the blue wires to blue, red to red and yellow to yellow. The black and white are connected to line. Only two of the tubes light. I have switched the pairs of red/blue/yellow but this does not help. I can not remove the fixture from the ceiling to look at the wiring to the sockets. Any suggestions?