Have you looked at any other "commercial" mixes? Done a side by side grow out? I would find something similar to what you are using from a different manufacturer, plant some plugs or seed into both mixes, and see what happens...process of elimination....

What goblugal suggests is the first and easiest step in the process of elimination you are going to have to do to find out what the unknown value is ruining your crops.

Is this a soil-less mix? If so you need to also suspect your nutrient source if you do a constant feed. You can only change one variable at a time. First try a different soil mix. Grow some flats side by side with your old mix and monitor. If they both exhibit symptoms, then move on. Next look to your water soluble or prilled fertiliser.

Are you using any sprayers you may have used pesticides in?

Think of air pollution too. Any industries nearby discharging out stacks? PAN damage is easily googled.

You aren't using old benlate are you? Years ago some was recalled because of herbicides showing up in the formulation.

What about your heating system? Do you have it regularly maintained to be sure it's clean burning, and not throwing out harmful discharge. Some natural gas contains impurities like sulfur and many growers are sitting on their own gas wells. Gas leaks into the g'house? Have it checked.

Send in some soil samples and/or plant material samples to your state ag pathology lab. It used to be a free service to us in our state if we belonged to certain professional groups, but you pay now. It's still cheaper than a year's profits.

When I was growing for another concern, the owner had gotten a 'good deal' on a bulk shipment of soil-less mix and it was sour and just reeked. I told him that there was something wrong with it, but he made us use it anyway. The resulting crop next spring was not up to par. Have you been using peat or bark based media you have had stored for eons, or perhaps has been stored and gotten wet repeatedly somebody else has stored outside?

Are you reusing plastic?