Depending on the length of your beer line I wouldn't go much above 12psi for any length of time. You said you kicked it up to 20, and that is fine for a day or two to see if the carb level jumps up some, but more than a couple days there and you risk over carbonating.
If you suspect the regulator, try swapping the regulators and see if the High Sierrra carbonation improves. Maybe swap the input lines to the kegs and set the High Sierra up to like 25psi for 24 hours and see where it is at after that point. You might want to check it at 12 hours just to be sure you don't over do it.
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