Sometimes things just don't work for no obvious reasons. Once you reached this point it's time to create a packet capture for analysis. Browse to Subscribers → Overview → Connected Devices and click the spider-symbol in the action column next to the subscriber in distress.

Upon click, the packet capture immediately starts with the proper settings. Once you think you are done let it run for another half minute just to be sure and the hit stop. You'll now be offered the opportunity to download the file.

The .pcap can be inspected using tools such as WireShark. It tremendously helps support if you can provide a packet capture when you are experiencing hard to troubleshoot problems or problems specific to a single/few subscribers.