As your plants age normally some roots die off.......this will lower the ph in your older soil. Things like Rhizotonic and Cannazym will assist with ph balance in this natural process but I have a feeling that's what's happening to your grow. It's not uncommon to transplant to fresh soil on a fairly regularly basis which will help keep your ph in balance and your soil well buffered.

Can you transplants to fresh soil and leave out the lime they told you to put in? I use two soils only....Happy Frog and Oceans Forest and I add nothing but perlite to either one and love what they do for my plants.

If your runoff is that low and you can't transplant, then you need to flush the acidic crap out. Check out Stinky's thread on flushing your soil/how and why. Once you have flushed them and get a runoff within the normal range, switch to distilled or RO, check and ph EVERYTHING that goes into your pots and just limp them along to finish.

Lastly....in soil EVERYTHING....both in and out should be ph'd between 6.2-6.8 so you're already adding water too low in ph. There's some of the issue right there.