A common reason why users, Quicklink Servers, ST500 (Studio-in-a-box) and other devices may not be showing within your device is due to having a filter applied.
Within the device list on the left-hand side of the Quicklink Manager, change Filter by to All.
If the device filter is set to All, and your Quicklink Server's are still not showing, this is likely to be the result of Quicklink Server channels not being logged in.
To resolve, or check this, open your Quicklink Control Panel within your Quicklink Switcher. Select a Studio channel, then click More Features. This will display if and who the Studio channel is registered to. If the channel is not registered, enter the details of a Quicklink Manager account, then click Register. Investigate this for each channel within your Control Panel.