Only think I can think of is that the supply lines are backwards...is there any way for you to check that? You might want to try to call Grohe. Well, if they installed the volume control in the wrong place, maybe that would mess things up, too.
I've only used a couple - one in my bathroom, and occassionally in a hotel and they've held their adjustments fine. There is an internal adjustment, but thatis only to make the knob temperature markings agree with the actual temperature. If I remember, the max setting is part of that - if the temperature is right, the max is right, too. Mine seemed pretty close from the factory, so I didn't adjust it.