I have a 2012 390Bh60 and fought with this exact issue until last summer.
The solution is as follows.
The unit is currently wired for low/auto. The auto switch is faulty .... as was the case with many of us.
Once you bye pass the auto switch the the unit with be high/low.
If you open up the outside access door to your water heater and look at the upper left of the back wall ... you will see the auto switch with a wire coming in and going out. If I remember correctly the switch is about the size of a quarter with two blade connectors.
If you remove both wire from the switch and connect them.together, you problem will be solved.
I also stopped using the small (low flow) barrel type pressure regulator and installed a high flow watt regulator that I purchased at Lowe's.