It is possible that there is a real problem, but first check for this:

The default sensor setting for the Go Direct^® Motion Detector (GDX-MD) is Motion. There are two other sensor settings:

Motion (cart)

Motion with TC

If you start up Graphical Analysis and collect a run of data using the default sensor setting (as 99% of all people do), you will see data on the graph and all is well. If you then change the sensor setting to either of the other two options, and try to take data, you will see a blank graph.

The issue is that the vertical axis of Graphical Analysis is still set to graph the default Motion channel. Click on the vertical axis and change what is displayed. Another way to deal with this situation is to just choose New Experiment.