Mac Book Air A1369 screen replacement

The original screen on my daughter's MacBook Air 2012 (A1369) failed a few weeks ago. I plugged it into an external Apple Display via the display port, and that worked fine. I was able to boot and login. Everything worked normally. Ruling out a problem with the graphics chip, I bought a replacement screen (the entire display assembly, not just the screen) and I fitted that as instructed.

And yet it still doesn't work. When it boots, the screen is blank. I then tested with the external display and that still worked fine. I've triple-checked the screen's connections to the motherboard. What should my next move be?

For background, in the past 2 years I've successfully replaced the battery and the motherboard. There is very little left of the original device.