Not sure on all the details, but the was my experience w/ a lilliput 7. First, try replacing the cable. Second if you are using windows 8, make sure you boot to desktop and not win8 ui. Due to these monitors being less than 1280x800 (the 16:10 of 720p) win8 would not load to then. You can google how to win8 ui on 720 and there is a way to change the registry to support it. For other OSs, not sure what the issue is.
To determine this issue, i brought a pc monitor out to the car and dual screened it. Boot to the pc monitor, log in, and go to desktop. The plug in the lilliput and extend desktop.