The assumption that distilled water allows pH to remain stable actually doesn't work despite the truth being counterintuitive.
Tap water contains calcium carbonate, and that in turn controls soil acidity. If you are in soil and your tap water isn't known to be horrible, use the tap water.
I don't know exactly which of the soils you are using but PEAT is a common ingredient and tends to dop in pH over time, especially after the lime added at the factory as buffer is used up... and guess what that is? Calcium carbonate again!
I think you have a lockout situation and likely your pH is too low too.
Complete a soil runoff test and FLUSH!!!!