Try the connection between another machine and each of the ones you are working with. This should help to isolate the problem.

It could be hardware-related, too. As a first step, you could spray some contact cleaner into the ports and cable ends — couldn't do any harm, and might do some good. Deeper down, there may be a problem with the power supplied to the ports and cables, since, after all, there needs to be current running down the wires.