I think it's safe to say you know how to grow.  You know how and when to feed and everything seems to be on point. I wonder if it's a root issue, like root bound.  Your plants sometimes get rreally big for the pots they are in and you pretty much veg and flower the same length of time so the only thing that really changes is your strains. When you harvest are the roots all you see top to bottom when you pull the cube out?