The flame regulator probably isn't functioning right- you should be able to get the thing to ROAR at high flow. IIRC this unit has the same mechanical feedback as a log of older European tankless units have.
Download the manual, pull of the outer cover, and see what the flame-control slider is working against. Usually it's a cam of some sort working against a spring-loaded (internally) valve/regulator. If the part that should be riding on the cam isn't come up, see if you can't gently tease it free and get it working again. Sometimes there's an adjustable limit-screw that can be tweaked in/out a bit to let it operate in a less sticky region.