external monitor not detected - Early 2015 MBA

I am the IT admin at my company here. Recently, one of the employees started having severe problems with his Mid-2012 MBA, which would only operate in Safe Mode. Because he needs his computer at all times, my initial "solution" was to give him a new MBA, then try to work on the old one. I gave him a brand new "Early 2015" MBA.

For one day, it worked without issue, but then this morning it stopped detecting the external monitor.

I have done the following:

1. Swapping hardware. I've tried this with a DVI to MDP cable, as well as an HDMI cable using an HDMI to MDP adapter and a DVI cable with a DVI to MDP adapter. I've also tries two different monitors, including a brand new one out of the box.

Note that ALL of these configurations work flawlessly with the 2012 MBA, but the 2015 one doesn't even detect that anything has been plugged in (it does not do the familiar "flash" on the laptop screen).

2. SMC Reset

3. "Detect Displays" from the Displays system preference panel.

4. Plugged another device (a hard drive) into the Thunderbolt/MDP. It works fine.

Both laptops have OS version 10.10.3

Any advice would be appreciated.