a few ideas come to mind, 1st it could be like you said and you have nute lockout due to a buildup of salts in the soil, try flushing them with twice the amount of water of the size of your containers...{taken your growing in soil}then add about 10 caps of 3%h2o2 to a gallon of water and add your ferts. the h2o2 will super oxigenate the water. i add h2o2 to every drop of water my kids get it helps super stimulate root growth making for super healthy plants! another thought is....how cool does your room get when its dark? low temps will also lock out nutes. given the time of year, and the purple stems that could be it? id have to know a little more about your setup before i could say for sure. btw what kind of nutes are you using? let me know if this helps peace