Why Won't My Board Reset?

I’ve got a slightly annoying problem with one of my dev boards. The reset button does absolutely nothing. These are the boards I got made by PCBWay — they did the PCBs and they came out really nice. I did the assembly myself. But somewhere along the way, something went wrong on one of them. A Tale of Two Boards I loaded both boards with a fast blink sketch to make the problem obvious. On the working board, hitting the reset button stops the flashing immediately — exactly what you’d expec...