it could be overwatering or underwatering what sand did ya use
not that found at the beach i hope !!!
could be malnutrician what nutes ya using
i would get some perlite in that soil mix to help drainage and airation to roots
beleive me this helps alot
provide a few more details and we will be able to give ya a more acurate
answer
what nutes ya using
what sand ya using
how ofter are ya watering
how hot is ya growroom
what light ya using
and how close is ya light
cheers