Hi,
that is a known bug that Tom fixed already for the next release (whenever that will be ).
Here is what happens: Currently the V/S scrollwheel is coded to have a limited number of revolutions up or down. So you can roll it down for maybe -10.000 feet...then it gets stuck. Whenever you select another mode, the number on the display gets reset to 0000 - so it is hard to see that you maybe already almost "used up" your "rolling down" number.
So one trick is to select another mode, then roll your V/S scrollwheel the "other way"...this frees up additional revolutions in the other direction. Think of a string attached inside...once you are at the limit, you can´t turn it further anymore. But rolling the other way will loosen up the string again.
Sorry for the inconvenience, Jan