Hi Billy_S,
Thank you for reaching out Microsoft Community. We're sorry for the inconvenience this has caused you.
Please check this link https://support.microsoft.com/en-us/help/4023496/surface-troubleshoot-connecting-to-a-second-screen for further troubleshooting. The link contains the information you need to extend Surface displays to external monitors. Please clear the display cache (See Solution # 5 for details) Once done, please make sure that the Surface dock's firmware is up-to-date. To check for firmware update, visit this link: https://support.microsoft.com/en-us/help/4023478/surface-update-your-surface-dock
If this helps, feel free to mark this post as an answer. Otherwise, don't hesitate to respond to this thread for further assistance.
Kind Regards,
Crest_Z