Anyone know what the problem is?

First I see you started nutrition too soon. When they were small. So overall I see now is likely too strong nutrition. Not enough run off likely and ph is off in substrate. I would measure the run off and depending upon substrate 6.4 for soil, 5.8 for coco or hydro type growing. Good luck and happy growing 🌱🌱🌱

That is nutrient lockout, probably a pH balance problem. If not then did you feed is nutrients? You could of over fed if you did and it's just to much for the roots to handle. They stop uptake and they just start to burn up.

Hello fellow grower. Looks like you ph is locked out test ph run off an apply from there up or down try to get soil in range of 6-6.5ph.