1 Color Palette ( Material Palette) is on null
(circle with line across it..change it!)
2 Wrong layer in layer palette is active
(The active layer is the highlighted on in the palette)
3Layer is turned off! (X is showing on the active layer palette)
(Click the x to re-activate it!
4 Opacity setting is too low (PSP8)
Find it on your settings bar at the top section of the screen
and change the opacity to 100%, then try again.
5 It is filling but can't be seen because a layer on top
of the layer being filled is blocking the view.
6 There is a selected area so small you can't see the change.
Selections/Select None...then try again.
7 You have not selected the Flood Fill tool? Duh huh! LOL
8 Lastly, you have not left or right click on the selected area to fill!