I'm using DWC bubble buckets and have been feeding them the lucas fomula with GH line of nutes. I'm thinking that I should have done some sort of flush when going from veg to flower.
I think you're on the right track with mag, the question is, is it from a straight def or is it an OD of another nute or a ph issue that's locking out mag.
i get this prob some times with my tap, i recommend adding half a vitamin pill to the water it will save you some cash. been doing this for a while for calmag works very well