Do either of your structure images have a transparent background? Have seen cases where that seems to confuse the model and filling in the transparancy with a contrasting colour could help. Have also seen a problematic image that was mostly back with some white shapes work after inverting the colours.
My images are full color, low res, jpg files. I do this already for some time with a lot of nice results, but now no matter which image I upload, I get the same image generated. Because I keep my prompt the same, but generate different images, changing the structure image. With no result right now.
I used Safari Version 16.5.1 (17615.2.9.11.7, 17615) and Chrome Version 125.0.6422.61 (Official Build) (x86_64) Both are not generating new images with different structure reference images.