Sorry for bringing up an old post, but I found this post when looking for something else and I had a solution. I also have had some similar issues that you did. Generative expand or fill would not work on the edge of an image to extend a solid white background, yet I could generative fill skin to fix blemishes in the bikini line area on the same image - didn't make any sense to me. I finally figured out a work around.
If you're trying to generate something that is on the background or not your subject showing skin, just select subject, and then content aware fill on a new layer. Keep working, then hide that layer and it works just fine.
If you're trying to generate something ON the subject, draw a selection around most of the person that is not the area you're working on and also content aware fill on a new layer. For example if you're working on a leg, get rid of the upper half of the person and it seems to work just fine.
I've also found it can be picky when it comes to things that are close to skin colored (like wooden cabinets). In that case, I put a hue/saturation layer above the layer I'm working on and change the color slightly, do your fill and then delete the layer. You may have to tweak the generative layer a little bit to get the colors to match, but that seems easier than going old school with the clone stamp.