It could be several issues, What nutes are you using and have you changed the dosage lately? Have you transplanted lately or done anything else that may shock your plant? WHat is you temp and humidity in your grow room?
Either over watering or low on N or PH issues causing lock out
When you switched to 12 /12 ??? did you fully switch to bloom nutes ???? if so i say give them girls half Veg and half bloom nutes next feeding
Or how long have you vegged for ??? and what kind of dosage did you feed them in veg maybe a good flush to rid salts in your medium then half veg n half bloom for a bit and monitor plant for any improvement