So if you start drawing in Flash, you are dealing with one of TWO different types of elements:
STROKES are basically LINES - certain tools like the LINE TOOL, PENCIL TOOL and the PEN TOOL are ideal to create lines
FILLS are basically BLOBS of coloured area. The BRUSH TOOL actually paints FILLS, and not lines.
The PAINT BUCKET TOOL can be used to FILL areas defined by strokes with colour, but there are TWO RULES that you have to follow to make it work.
Like a swimming pool, if there's a leak in the walls, it's not going to hold the contents. You have to make a SOLID PERIMETER all on the SAME LAYER for the area to be filled in.
Your best bet is to:
Use SNAP TO OBJECTS when first drawing your area. It will help you close the perimeter more easily.
ZOOM IN on the stroke and use the HAND TOOL to inspect around the perimeter to find any holes to fix
There IS a "Gap Size" option at the bottom of the toolbar when you use the FILL TOOL. It CAN be used to adjust the tolerance so that small gaps won't interfere with the FILL TOOL.
Make sure OBJECT DRAWING is NOT on while drawing the perimeter.
Use CTRL-A to select all objects, and make sure the perimeter is made of just one, solid stroke. If any blue "bounding boxes" appear, it means some elements have been grouped. Fix it by using CTRL-B to break them out of their groups an allow them to interact with the other strokes in the perimeter