GIMP odd behaviour Some parts don't work

[Ofnuts]

Another possibility is that the active "drawable" is not a layer (which is RGB) but some grayscale object, such as a channel (typically, if you Select > Save to channel the active drawable becomes the saved channel until you select back a layer) or a layer mask.

For more clues we would need to see the whole UI (in particular the Layers list and the status bar at the bottom).