yeah, i'm not an expert man and i've never actually used
promix but if i remember right it's peat based and i know that ph can be an issue with peat, ...and ph can exhibit kinda like this.
i have to say though that 400 - 500 ppm in flower is very light feeding, especially if the
promix is indeed peat based and unamended, ...i myself grow in coco hempy buckets and coco is also quazi inert so i water and feed daily, no water water feed, just feed feed feed, lol, and i feed at about 600 - 700 ppm.
if this is a recurring problem then it won't hurt to try feeding at every watering or every other watering, for at least one plant, just to see if it is indeed just hungry or if you have an actual issue.
peace, bozo