I’ve had this happen with more than one image lately.
I open an image in GIMP with the purpose of making the background transparent. The first thing I do is Add Alpha Channel. I proceed to remove the background. Once I’m done with that, I export as PNG. But the background isn’t transparent. I can’t figure out for the life of me why that is. I’ve never had this problem before. The image is in RGB mode. I’m using GIMP 2.8.10. Is there some obvious step I’m missing?
I’ve worked out why this might happen. If the Image Mode is set to Indexed Color, then the export doesn’t seem to accept the added alpha channel. So:
Image → Mode → RGB
File → Export As → imagename.png
This should now export the image with the alpha channel, and transparency.