G'mic inpainting doesn't work on iPhone images

The use of Gimp G’mic to remove an unwanted feature in an image is very effective with photographs taken with a traditional camera but I have recently found that it simply does not work with images from an iPhone. I have tried using the normal technique of covering the unwanted item with a pure colour (usually red = ff0000 but other colours too), selecting the coloured area together with an adjacent replacement area and activating G’mic >Repair>Inpiant (Multi-Scale). Working with a small area normally results in the unwanted item being rapidly replaced. With a similar situation on an iPhone image G’mic does nothing.
Can anyone explain this and, hopefully, offer a solution so that I can retouch my iPhone images too?