Is the Image in Indexed Mode ? ie. What does Image -> Mode show. If it is Indexed, then change to RGB
Otherwise
Post a screenshot of your whole gimp interface showing layers and tool options as well as any active selections.
EDIT: Starting to guess about your image but if the shapes are on separate transparent layers then lock the alpha channel before filling. A quick example https://sendvid.com/b9mbddww
Otherwise
Post a screenshot of your whole gimp interface showing layers and tool options as well as any active selections.
EDIT: Starting to guess about your image but if the shapes are on separate transparent layers then lock the alpha channel before filling. A quick example https://sendvid.com/b9mbddww