Your soil and nutrients are the culprit. MG nutes tend to lower the PH to very unacceptable levels. If you refuse to check the PH you are leaving out a most important factor for your plants. The problem with flushing MG soil, is there is TIME RELEASE fertilizers in the soil. You may have flushed out the excess nutrients that you provided your plants but your soil is still going to harm your plants. This is not to say that you can't still finish the plant with good results. I have done many grows in MG soil, but I am VERY careful about adding additional nutrients. I would do another heavy flush, and use straight water until you see a stop to your lockout problem, also,
¿get a PH reading on your runoff?