Thanks for posting!
We actually don’t recommend our UD-3900 dock for Mac systems, as there can be performance issues (we do our best to call this out in our product listings). That said, many of our customers do in fact use the UD-3900 with Macs, and the behavior you describe is not typical.
One quick question for you, can you confirm what specific brand and model number monitor you are connecting to the dock, and what type of video cable you are using (what type of connector is on each end)? I ask because in some cases the video cable can be the cause of the problem and this information will help us rule that out.
Pending that information, one quick thing to check is that the video source input on the monitor is set correctly (using the buttons on the monitor) given the type of video cable in use. This is important because in many cases monitors with multiple inputs will not switch to the preferred one automatically.
If checking the input does not help, please just let us know the additional information and we will determine the next steps from there,
Thank you for giving us the chance to help!