It depends on the light in the photo, things in the foreground/background, & often where the focus was.
things that just aren’t as pronounced in a darker or especially busy photo, just don’t show up for the remove background feature. You’ll notice some pictures, It works for with everything and others, it does not.
also, pictures that you may have used mark up on, will prevent it from working.
If you’re just finding this frustrating every now and then, notice the light in the amount of contrast for the items. One time I edited the contrast in a photo which had my mother off to the side, and after I change the contrast for clear definition, I could lift her into the sticker, whereas before I edited the photo, I could not. That was just a fluke but helpful to know. I hope this helps.
*btw that feature is not available on older phones or systems. Everything Has to be newer than the XS iPhone. That’s because the capability comes with the with A12 Bionic chip only available in later models & works in Photos, Screenshot, Quick Look, Safari and more.