I would agree with FoTwenty. That looks like nitrogen tox, aka the CLAW. if left untreated, the buds will remain small. As mentioned, flush once the medium dries out.
Some strains HATE Nitrogen once they hit preflower and flower. Some just hate the N balance in a given N-P-K feeding mixture.
You should be able to fix this no problem though. They will recover!!
Happy growing!