I can't give you a definitive answer but I can run through a few tests I would do.
1) Change out the bulb that has the minimal output. I have seen multiple ballast reactions to bad bulbs and have seen this before.
2) Check your common to that ballast(white wire)
3) Did you use different input wiring for the fans. (I wouldn't link to ballast power supply).
I really think it is the bulb hopefully not the ballast.
Good luck on the tests.
Kevin
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