Hey bud do you own a EC/PPM pen or know of someone that does?
When flushing your focus is the EC/PPM. so having a way to monitor and take readings on that will give you clear guidance instead of having to do the guessing game.
You fill a bucket out the tap, read the EC, write it down. (should be low, below 1.0, if not don't use, get 5L bottled waters) Now use that water and slooooooooooowly get the pot to field capacity (this will ensure no dry pockets, so your EC reading will be true and more accurate) once you get first few drops of run off you can give a nice drench to get a good amount up to 500ml running out the bottom. Read the EC and see if you still need to flush or not. With living soil a mature plant can handle around 7.0EC and seedlings around 1.0 EC, you play between that. Synthetic growers never really need to go over 4.5 to 5.0EC.
The flush may have resulted in a too low EC medium I grow in 30L pots, have had runs in the past toasting my plants to a crisp then doing a full 180 right by the grave. EC read out was 9.9 (anything higher my probe can't even read) but I only flush with like 5 to 10L of water then I'm back at 0.0EC then I have to rebuild the soil with Jamies Elemental blend, bat guano, gypsum and Diatomaceous Earth.
Nute lockout usually happens cause of salt build up, that can happen even in the earth, not only salty synthetic hydro growers. organic living soils are not free of the salt build up threat. It happens as a result of pockets of nutes, which is something completely natural. the rhizosphere you have a whole micro-universe, if one strand of root finds its way into a pocket of nutes that's too much for it to uptake the root will burn. This will show as early onset signs of nute burn on the leaves, but what happens in the rootzone is the pocket of nutes never gets uptaken cz the roots around that area is now damaged. Now the nute compounds find eachother, same way if you put too much salt in water and let it sit, the salt will "find itself" and recrystallize. This happens on such micro scale you wouldn't be able to see physical crystals with your naked eye, but they're there, these chain of events is what we call "nute lock out" cause the roots got too much of a certain compound and the result is locking out that compound completely, majority of the time you can also see the plant stops uptaking any nutes for that matter or even sometimes you get like N lock out but the plant indicates a Ca shortage, cause it's not always the compound that causes the lock that'll be the one locked out, if that makes any sense? It can also show as nutrient deficiency at later stages. You keep feeding and it seems to not do anything. It's like ph zones, if you out of the right zone the plant only uptakes certain nutes and doesn't uptake others. so you gotta find that fine line balance.
Anyway, hope any of this helped at least a little.