Either the monitor reading is inaccurate (very common) or they left the cutout of the tank in the tank itself. This happens when they drill the cutout for the toilet and the remining disc drops in the tank and can block it.
If you can physically get to the side of the tank, tap on it and see if you can tell if it's really empty.
If the monitor is reading bad there are a few methods to try and clean the sensors, the easiest is a black tank flush.
Or - they have the dump valves for the tanks labeled incorrectly. Your black lever is really your gray tank and vice versa. In the 4 MH's I have owned, 2 had this issue.