Thunderbolt 3 Dock only working with 1 HDMI display at a time with 2017 MacBook Pro

I bought this for my 2017 MacBook Pro…

Everything is working except I have 2 HDMI Displays and it’s only working with 1 at a time. One display is going from the dock -> DP to HDMI adapter -> monitor. The other is dock -> USB C to HDMI adapter -> monitor.

Product page states: “Mid 2017 iMac and Dell Thunderbolt 3 systems can support an additional USB-C Alt Mode video adapter (sold separately) to provide a second display”. I bought this for the second display:…

What works and doesn’t work:

-If I leave both monitors plugged into the dock, only the first will work.
-If I unplug the first monitor from the dock, the second one will work.
-If I unplug the second monitor from the dock, the first one will work.
-If I unplug the second monitor from the dock with the USB C adapter and plug it straight into the MacBook, both will work fine.

So clearly the dock supports both adapters just fine, but something is stopping them from working at the same time.

Hi Thomas,

Thanks for posting, and I would be happy to help with your new docking station!

Thank you as well for the detail you provided and the troubleshooting you have already performed that is most helpful. One quick note, we have not tested the USB-C video adapter you mention so we cannot guarantee its performance. Not saying that it is a contributing factor here, but I just wanted to make mention. Plugable makes a USB-C to HDMI adapter ( and USB-C to HDMI cable (…) that have been tested with our dock.

Moving forward, based on your description of the behavior our next step is to get some additional information. If you would while everything is still connected to your system (even though one monitor is not working), please follow these steps:

  1. Choose the Apple Menu -> About This Mac.
  2. From the window that appears, click on the ‘System Report’ button.
  3. A System Information window will appear. From the application menu bar, click on File -> Save… to save the information to a file.
  4. Send the file to with ‘Ticket 211724’ in the subject line so we can match everything up. Please also include your Amazon Order ID for the dock purchase in your email (please don’t post that in our public forum).

Thank you for giving us the chance to help!

Plugable Technologies

Was this issue resolved? I’m having the exact same issue. 2017 Macbook Pro, connecting to dual hdmi monitors through DP to HMDI adapter provided by Plugable and USB-c to HDMI.

Hi Dale,

Thanks for posting! In Thomas’s case the issue was determined to be the Thunderbolt 3 cable being used to connect the dock to the system. If you are not using the Thunderbolt 3 cable included in the box with the dock to connect it to your system, please do so to see if the behavior changes.

If that did not help (or if you were using the included cable already), our next step will be to get some additional information. While the dock is connected to your system with both monitors attached (even though one is not working) please run our PlugDebug utility ( and send the output along with your Amazon Order ID number to and we will be happy to help.

Thank you,

Plugable Technologies