Stunted roots never reached the reservoir

I have done a number of successful grows, but this is my first attempt since 2020. Using rock wool and the old BCNL Bloombox. Seedlings were healthy. Week one in the box, strong white roots grew out from the rock wool but turned brown and never reached the water. Meanwhile, plants look green and healthy but I’m assuming they are growing more slowly than if the roots were healthy. I assume these plants won’t flower and bud successfully. Only change from earlier grows is the purchase of new chemicals (general hydroponics) as the ones I originally got from BCNL (which worked great), got too old. Any thoughts on what might have gone wrong?