Sounds to me from the tips going brown that the soil is too "hot" i.e theres too much fert in the soil for seedlings, especially if its a sat or sat dominant strain as they dont like much fert and most will grow quite happily with minimal added ferts.

Try starting yr seedlings in a small pot or foam cup or whatever of light mix soil for seedlings and cuttings, then transplant into the stronger FF soil after a week or two.