The GPU wasn't fully pressed into the slot of the A3. With an ITX motherboard, the cables from the front have to stretch a bit further, creating tension on the backside of the GPU that make it a bit harder to click it in on the right side of the PCIe slot, near the RAM/PCIe clip. The slam-dunk diagnosis was that the customer was able to remove the GPU from the motherboard without having to unlatch anything, which meant it was never latched in fully. I encouraged him to press hard in the middle of the GPU, and he heard a click finally. The GPU wouldn't slide back out this time, indicating proper seating/retention, and it booted and operated correctly.