When you flushed, what was the reading on the runoff pH?
And pre-wetting the new soil isn't really a great idea. The roots aren't in there yet to use the moisture, and it jsut SITS there and stagnantes and creates conditions (aqueous) for any chemicals present to interact in pretty unpredictable ways.
Pot drainage- where are the holes in your buckets? 5 gallon pails have a lip around the bottom edge that prevents draining if all the holes are in the bottom. You say you can veg fine but not flower; I assume you use large pots only for flower? Drill holes in the SIDES too, big ones, like 1/2" or more. You GOTTA get oxygen to the root zone.