I had the same looking stuff on my first indoor grow. First my soil was too dense, second I was severely under feeding them. I thought it was lock up as well, started cal mag , and continued to see yellowing edges, and small copperish spots that ran thru the leaf...not saying this is what you have...but beware with all your flushing....these babies need quite a bit of nutrition to grow like weeds. Be sure you are checking the pH of your run off water fairly often as well.
Rapidly working my way up to a full strength feeding program eliminated all the problems. I guess overwatering and nute burn can appear similarly in the early stages in regards to yellowing leaves, and spotting.