Have you tried flushing?
I have had several babies that were having PH in the low 5's!! I took water with a little higher PH than normal, like a 7 instead of a 6.5, ran about 3-4 X the water as the pot size and by the time I was getting about 2/3 I check the PH again to see if it is going up, as I am getting closer to a more normal range of run off I start using water at my regular PH level. This has worked for me, it takes a lot of water and I was afraid of it being too much but they were ok, I hold off nutrients till they have a 2nd water with just plain water and the PH is not lowering like it was.