yellow leaves in flower half way up the plant

I'm an old timer and I am stumped. I have been doing things the same way for years with consistent results. Lately, maybe last crop too, I am getting a lot of yellow leaves half way up the plant. I am used to looking for yellowing at the bottom when they are hungry and at the top when something is out of whack but I'm stumped on what is going on in the middle. I just thought of one thing, and they are all new varieties for me. I guess if I had my old trusties in the room I would know whether to worry or not.

I may have answered my own question. It may be a trait of these strains; Blue Dream and Incredible Bulk. They are very green at the top so I don't think they are underfed. I usually feed light anyway, 5-600 ppm. I guess I could give them more next time around for a test but they are pretty deep green already at the top, greener than i'm used to seeing.

Oh yeah, it's DWC and Dynagro Foliage Pro. PH drifts a little but not bad, sometimes a little high and sometimes a little low.

Oh yeah, I grow 5-6 ft trees with 5-7 zips per plant. Small plants wouldn't have much of a middle to them.

A picture would help..

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without a pic theres no way to say