Personally, I'd guess temperatures, but CFLs will lose lumens over time. 6 grows using the same lights throughout veg and flower? if so then

veg @ 36 days x 24 hours = 864 hours x 6 veg periods = 5184 hours
bloom @ 60 days x 12 hours = 720 x 6 blooms = 4320 hours

Add in some wear and tear for turning them on and off and you're at about 10,000 hours on those cfls - getting pretty old. I would imagine the output is significantly less than when they were new. Get a new bulb and compare them side by side. Or just get all new bulbs and rule that factor out - they are cheap and the ones you have seem to have done plenty enough to be "retired" or used as spares