I have had many instances where the detection of the TUDS after resuming the computer from sleep has failed, however, plugging the TUDS into a second computer, or in some cases into a different USB port, will show the dock to be active and functioning.
In all the cases, bar one, this has been resolved by updating the computer systems hardware drivers.
I found one case where one variant of the 2nd Gen X1 Carbon did not respond to driver updates. The version 20A7-006JAU required an update of the dock firmware on models ACP71 and ACP77 to version 909x.
As a comparison the version 20A7-CT01WW did not respond to the updated dock firmware and was fixed by driver updates as detailed in a previous blog subject OS Downgrade – And OEM Driver Packaging
If you complete the Lenovo recommended driver updates and the issue persists please call your local Targus support team to confirm the issue and provide the 909x firmware update.