CrossRoads:
Vin needs >= 6.5 to 7V for the onboard 5V regulator to work. Otherwise feeding 5V into the 5V regulator does not result in a workable 5V coming out of the regulator.If you want to power the board from 5V, connect the source to the 5V pin, or power it via the USB connector.
Thank you! I didn't realize I was powering it from the wrong pin. It works now.