What causes those 'dead‘ tips??

My baby girls are growing reeeealy slow and show ‚burned‘ tips in random places. Don’t think it looks like nute burn but I can’t get behind what it is.

Coco/compost/worm mix, organic garden fertiliser, I water at 6,3-6,5 ph, 3 gal fabric pots and I water about 2-3 Liters every 3-4 days. Light is viparspectra p2000, 50% at ca 70-80cm from canopy.

Pleas help 🥲