Hello Sam, thanks for posting and I am sorry to hear of this issue! Just to clarify, the monitor that is not working when connected to our dock seems to be following just the HDMI port while the monitor connected via DVI works without an issue?
Assuming that is correct, it could be that the HDMI port on our dock is faulty or there is some software issue occurring. As you have already tried reinstalling the DisplayLink drivers several times we will want to examine some log files from your system to help determine what could be happening.
To help us do so please download and run our diagnostic utility PlugDebug --> http://plugable.com/support/plugdebug with everything connected and while the 3rd monitor is not displaying. Please send us the file that is created on your Desktop by attaching it to an email to our direct support team at email@example.com with ‘Ticket Number 190709’ in the subject line. We’ll match things up and determine next steps from there.