9550 - Thunderbolt 3 docks not connecting

Hi,

I have the XPS 15 9550. I bought a startech thunderbolt 3 dock. It was completely dead - no sign from the laptop when I plugged it in. They replaced it. The new one worked fine for a day. Then it too was dead - no sign that it's connected. It seems unlikely I had 2 dead models. Any ideas whether there is something on the PC that could cause this? BIOS is updated, as is thunderbolt firmware & drivers.