2 gallon coco sog grow 5th week of flower advance nutrients ph perfect leaves started to do this ph blue lab ph pen no longer wants to calibrate I ordered a ph doser instead I’m thinking cal/mag and a ph issue
Id definitely verify ph asap.
Picture of whole grow helps. Ph is always a first look. Then go for nute levels. Ph is a big killer in our grows. If you are solid ph in and out start looking at some mag or calcium. Then proceed to nute levels. Do NOT panic adjust anything. Slow and steady steps to get in line. A fast GUESS can killem. Give us a photo of whole plants and grow. The help will be easier if we have more visuals to start. But as I mentioned…start with simplest solution…PH…first then step by step till problem found. Just suggestions.
@ChittyChittyBangin is on point for starting repair.
Excellant step by step advise. We run through the same sequences at work troubleshooting machinery. 
Ty @PhatPat . A common misconception of newer growers is fixes are quick. Problems can be quick but even the simplest fix takes days to see. I always suggest to realize that a problem that appears over a 3 or 4 day period and caught can take twice that to see results. Aggressive guessing and a hurry up and recover attitude is counterproductive IMO. Especially in an auto that cant be babied forever. 4 month grow average auto. After more info this should be an easy dial back in.
I’ll take a picture of plant today