Again, they should not burn out after 6 months, got to get that figured out. Are they getting ventilation?
There is a difference of 48" fixture/bulb. If normal output (NO) it's 39watts a bulb, if they're high output (HO) then its 54W so quite the difference. The HO is what you should be running.
If you're not getting the growth and have the HO could consider dropping the actinic and going all 10K if want the whiter light. RC has a
huge thread on bulb combos.