Having trouble getting transparent backgrounds in ChatGPT images

That’s what I noticed on my image (in image gallery that I’ve linked ) that there still was little bit of the chessboard background. But the lion in this thread, was a success and also ChatGPT said that sometimes it works cleanly, sometimes not. Maybe it has something to do with complexity of the image?

@_j if you want, I’ve shared a python code on that image gallery, to test if you maybe get better result with it than me?