No sensei here!
Lockout can happen even if you're watering to runoff. It happens when there is too much of one or more elements in your medium that keeps the roots from taking up another nutrient.
Here's Mulder's Chart:
"For example, adequate potassium aids in use of iron and manganese, but if it is too high it will hinder utilization of magnesium, boron, nitrogen, phosphorous and calcium."
Source
So potassium locks out nitrogen. It was one of the main issues with MC, since folks would add more to keep their lower leaves green but end up with too much K because of the ratio out of the bag.
Since you're in coco flushing is a pretty simple affair (with a wet dry vac!) so that's seems like a good place to start. Don't forget to feed right after.