Can't Paint


I have a scene in Toonboom Harmony 15 (Essentials) that I an unable to colour using the paint bucket. Some (very limited) ares of a frame will colour but others will not. There are no gaps and the image was drawn using the pencil tool. However, if I convert all of the pencil strokes to brush strokes I am able to colour just fine.

Does anyone know why this may be the case, thank you very much.