There have been a couple of discussions here about gas deteriorating the inside of the hose and causing a "black" oil substance building up in the line.
I have found it in some of the tubing, but never blocking the line.
What did you mean by "Replaced the red regulator". The only thing that came to my mind was starboard side. Possibly separate regulators.
Since your tanks are on different sides do you have one long pigtail and one short to the regulator?
Possibly some of this oil blocking some of the pressure?