Do you ever reuse soil? I do, and I am having a problem currently from a mix I made last year. I threw some mater transplants in it because it's super airy, and they almost tripled in size within a 5 day peirod, however I think that because the mix is primarily comprised of coir, and it's a reuse, the Mag is depleted and I feel it has held onto some residual salts. I've recently re potted into sunshine mix #4 with 1tbsp/ trade gallon of fine powdered lime. It's been 2 days now so well see. But their still growing daily, even though there mottled. Early stage Mag deficiency, ie... interveinal chlorosis.
And now to my point regarding your post, I have come to the realization that a soil test is critical for this mix to actually perform. I'm a organic guy and a soil test is the only way to know exactly what's off. I have college university's where I live, in a big city where I can send mine too.(UW MADISON ) and they charge around $ 25 for a basic NPK test. Something mabey you'd want to look into. Soil can get very expensive over time