Alot of attitude here. Yall need to burn one and calm down.
Basically, even if you ph your water to the perfect range every time you water your plant, it is taking different things from your soil at different times. As things are removed and added to your soil, the ph of the soil can change drastically from the ph you are putting in.
I ran into this problem a couple times now. The plant is doing amazing until flower starts, then ph issues, wth.. well, different nutrients uptake at different ph levels. Even with my soil ph levels at 5.6, for veg it didn't seem to notice, so neither did I. I noticed at flower, when my plant noticed. I learned to monitor runoff ph and ppm throughout the entire grow. Dont wait for problems. Even if its just once a week, check it!
Thats when I started reading and watching everything I could about runoff ph. I'm no expert, but there is easy to find info.
The stuff your seeing deficient (P) could very well be in your soil, but the plant can't take it in because the soil PH is off.
First thing we do is Google the leaf issue and we get pages of deficiencies and toxicities. It could be. But before we know if or why, we have to make sure our roots are able to have it in the first place.
Soil ph, moisture level, the soil itself, the container its in... We don't grow marijuana, marijuana does that on its own. It does that through its roots. Our job as growers is to grow the best, healthiest, happiest root system we can.