Ingoing water ph for soil grows (including peat moss blends) should be in the 6.3 to 6.8 range. Above or below that, and you'll develop nutrient lock-out.

Your ingoing water ph is too low, and the 'claw' effect is your first indication. I adjust my ingoing water to 6.8 ph. Peat moss mixes will become more acidic over time, as the buffers fade.

What is your water ph before adjusting?