Do all the musicians say the same thing on the same day (ie it’s either all good or all bad, not not a mix of good and bad on the same day??).
Are you utilizing ambient and/or crowd mics? Is it possible that the audience sound was different on the good vs bad days? (ie some days it’s too soft or too loud, etc??).
Is the band locked in better than normal on good days?
You really need to press the musicians when they say “today was a good day” and find out WHY it is better. They need to be as specific as possible to help understand what changes week to week. Honestly I don’t think you will find that the “system” is changing, but rather some other variables (some of which I mention above, but there are lot of other variables it could be as well).