If you are only getting a single display to work with your Thunderbolt™3 device, make sure that your Thunderbolt 3 host device can support two separate DisplayPort lines over Thunderbolt 3.
While Thunderbolt 3 supports dual monitors over a single connection, not all Thunderbolt 3 host devices are designed to meet this requirement. If your Thunderbolt 3 host device does not meet this requirement, you cannot troubleshoot this issue with software, firmware, or driver updates, and you cannot use this device to achieve dual displays over Thunderbolt 3. If you are not sure whether your Thunderbolt 3 host device supports dual monitors, contact your Thunderbolt 3 host device manufacturer.
If you confirmed that your Thunderbolt 3 host device does support dual monitors and you are only seeing a single display, see the following FAQ: https://www.startech.com/faq/thunderbolt-3-update-software-firmware.
For more information about Thunderbolt 3 host system and motherboard compatibility, refer to the Thunderbolt 3 Compatibility and Troubleshooting guide.