How are you 'flushing' this pot? Most people who flush use 2-3 times the size of the container, and i hope you're not doing that.
I'm not sure if anyone has suggested this yet, but maybe you should add a tablespoon of crushed dolomite lime per gallon as a top dressing which will help with any cal/mag issues and keep your soil at a good ph once flowering starts. I usually add a tablespoon or two per gallon to my FFOF when I am mixing it up (also about 10-20% perlite but you can't add that now). It takes a couple weeks to start breaking down and being useful.