Do yourself a favor and get on that PH. Mg and Zn become unavailable in different PH ranges. Mg is good from 8 down to 6 but below that availability rapidly drops. Zn is good from 5 up to about 7.2 then it's availability falls off. Unless it's some kind of rootbound lockout, it's likely 1 or the other I'm guessing.

Really shouldn't be Mg since you have been feeding it. Could be Zn, Zn works hand in hand with Mg within the plant producing enzymes and stuff. Could manifest itself as both maybe if only one was missing?

How long has the same plant been in the same pot? Have you ever flushed?