Yes, soil is your best bet for buffering capabilities. So, if I read correct, you are feeding/watering at 6.5. And you are checking that after adding nutrients, correct? Are you checking your runoff as well? That would be a good place to start to see if the soil is out of whack. If your ph isn't off, the only other thing I would reasonably think it would be is something is locking it out. Whether it is an excess of phosphorus, which can lock out Cal or something else.
I am sitting here thinking what is the best way to approach it. It isn't bad yet, so don't start panicking and dump gallons of water all over it! So, if you have some micro nutrients for the time being you can foliar feed with, that is where I would start. That will get some much needed nutrients in her without having to rely on the soil. The second thing I would do would be, on my next watering, use a perfectly neutral water, preferably a couple of gallons of distilled. Do not change the ph, it should be 7.0, check it with your meter. Make sure your meter is reading 7.0, if it is off then we have a starting point. If not, water as normal and catch some runoff and measure it. Write it down. Also, after about 15 minutes, put a tray under your pot and from the top, squeeze down on your soil a little bit. Not like a hammer but just enough to get a half cup or cup of water that has been sitting in the soil. Check that ph and see if your soil is out of whack. From there we will see what to do. PM me if I don't check back in here when you have your soil ph. I forget a lot of stuff.