What this generally means is that you've queued the material in question for a construction or building, but the dwarves haven't gotten around to it yet. The only one "allowed" to move the item is the builder who's going to use it to build the whatever.
In this case, forbidding / marking the stone for dumping is actually preventing anything being done about it, because the stone is limited to the building until it is finished - and as long as the stone is marked for dumping, it will never be started. This awkward chain can only be broken by canceling the original construction that requires the stone.
The giveaway in this case is that trying to build in the same square as the recalcitrant stone causes it to not appear as an available option.
Channeling the square the stone is in is likely to result in a "construction canceled, item misplaced" message, but will solve the problem if you can't figure out which building wanted the stone in question.