To be honest with you I don’t flush and haven’t since around 2011.
but that’s not to say that flushing doesn’t have its place.
I believe you may be experiencing lockout due to over use of PK boosters?
have you used any? PK13:14? PK9/18? Or any other high P and K boosters?
With the upper leaves having issues it would lean me to thinking that you possibly have an immobile nutrient issue caused by either lockout, or over use of certain products.