Allsrolls I am NO soil pro for sure but I've just started doing some soil stuff myself and use pretty much the same formula you do....FFOF and perlite....whiich I love by the way.

I am discovering several things in my stumbling alongs here. One...I don't check the ppms of my soil.....in BRAND NEW FFOF soil I tested ppms one day and the runoff was something like 800. That was with RO water btw so that was ALL from the soil itself.
I DO check my ph both going in and coming out btw and good for you to check it as well.

I am discovering that my soil runs out of nutes VERY QUICKLY and am wondering if its possible you NEED TO FERT MORE. After a discussion with my hydro guy I have begun watering with nutes solutions two weeks after transplanting into fresh soil and water with a nute solution of 900ppm/ph 6.5-6.6 EVERY SINGLE WATERING. After about 6 waterings or so I notice my runoff ph start to rise and then I'll either flush or water with plain water until I straighten the ph runoff back out and then begin watering with my nutes again.

I am right in the middle of trying this fert schedule on my soil plants so can't say whether successful or not as of yet.......but I AM noticing less yellow bottom leaves that I tend to get now then before and I found that MORE fertilizing (observing carefully of course) has SO FAR led to healthier soil plants at our place.

The other thought is a rootbound issue.....yes you have big pots but you also have big plants. Are there any roots growing out the bottom of your pots?