Cant get coco ph to raise. Stuck at 4.0

Hey everyone. Long time coco grower here but having a serious ph problem last 2 rounds. No matter how much i flush through them ph stays between 4 and 4.5. I have tried to add gypsum to raise it and though it did help green them up, the runoff ph is still low.

Im thinking i got some shitty unbuffered coco or something, but not sure. Dont know if i should keep flushing them or maybe try some dolomite lime. Any input is welcome.

New grower here on first grow (in soil) had a little fungus gnat problem and this has got me to try coco for my second run. I am interested in how this is solved incase I run into issues. Hopefully you can get this solved friend!