If I can offer my two cents even though these guys have much more experience...
I have a very similiar medium in BushDoctor's Coco Loco by Fox Farms. Its is coco coir based with palm hummus and very light earthworm castings and bat guano. I sufferef the same issue with very little growth but didn't have any ph issues. I stopped by the local hydro store that i bought the medium from and he told me that the nutrients were only for the seedling stage and that I had to add nutes after the seedling stage. I bought some cal mag, Fox Farms Grow Big, and some super thrive. I have been feeding for roughly ten days upping the nutes with every other feeding and they have exploded. You may want to try this.