Can someone help me diagnose why my girl has yello...

You've got an iron deficiency, my friend. You can tell because as you stated the new growth is coming in yellow that's always the first sign. An iron deficiency is usually caused by problems with pH, though sometimes it can be triggered by a stressful environment and may clear up on its own after the period of stress is over. I'm thinking that the ph is too high at the roots and is getting locked out. Just remember to keep your ph between 6.0-6.5 and this should fix the problem. Now if you're using R/O water you're going to want to add a Cal-Mag supplement because R/O water has had all the iron and things like that removed from them, unlike tap water which still contains them. Hopefully, this was helpful and Good Luck with the rest of grow ...