How late into flower is it? If it is late in flower then I would not be worried.
It looks like Nite and cal mag to me, but probably due to lockout not lack of the nutes. There realistically should be enough calmag in the FFOF and the dolomite lime (more on that later) to get it through the whole grow. Cal mag is a trace element so not a lot is needed for the plant.
Also the PH that really matters in soil grow is your run off PH, you maybe putting in 6.5 but if you got salt build up from the nutes in the soil it will greatly lower the soils PH meaning you get nute lock out. It also takes time for the soil PH to change, which is why some people flush. with just the bottom leaves impacted not the new growth I would to suggest the flush. To get your run off PH just capture some of the run off water and PH that. If you want you can also go all out and buya super expensive soil PH meter, but they are motre for outdoor grows in earth where you cant capture run off.
The best tip I have ever received for my soil grows is to make sure you mix dolomite lime in your soil in the begging and top dress it start of flower and mid flower. It is a PH buffer for soil and basically keeps the soil PH consistent; what ever PH I water at is the PH that comes out. It also has Cal mag in it, so will supplement the FFOF has naturally. Super cheap and has made growing a breeze compared to when I didn't use it.
