Check the status bar to see if there is a message telling you what is happening.

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	If you add the Error Console to one of your docks most of the warnings will appear there. This can make it easier to spot any problems.

Check the Tool Options and make sure that all settings there have expected values, or else try to reset to default values.

Check which Image Mode your image is using. Some operations have limitations when using Indexed mode.

Check if a selection is active and if needed remove the selection.

Check the Layer Dialog and make sure the correct layer is active, that the opacity, blending mode and layer attributes are set as expected.