This can happen for a few reasons. The corrosion may be too heavy (like heavy green corrosion or heavy rust) and it would need to be stripped with an abrasive to get down to good metal. Flitz has no acids or abrasives (this makes it extremely safe to use on hundreds of surfaces without damage).
If you are trying to clean brass or copper, then you need to consider whether the piece has ever been lacquered. Most brass and many copper pieces are lacquer-coated during production because these metals are quick to tarnish. This lacquer or clear-coat finish can break down over time and the small holes and cracks that develop allow moisture and air to get through to the metal, which produces the tarnish. Flitz has no acids, so it cannot remove the lacquer. The lacquer must be removed with a stripper and then the metal is ready to polish. You should only strip the lacquer if the piece is solid brass or copper (or whatever metal it is). If the item is only plated, then a stripper will remove the plating along with the lacquer, and you will have nothing left to polish.