Ok, so I tried again, and 3 of the 6 machines worked. So I figured it can't be a configuration thing. We have to image using switches as we don't have enough drops, so I moved the machines that didn't work onto the switch ports of the machines that did work. Low and behold, they worked properly. So it sounds like I have some issues with my switches. I thought it could be throughput of the switch, but then I tried 2 machines on their own yesterday and they still didn't work.
Looks like it was a network hardware issue.