The Black Cats, who have had two bids rejected already, are believed to have made another offer in excess of ?12million.
But Wolves are believed to be now valuing their Scottish striker at nearer ?15million and as such have rejected the bid.
Wolves chief executive Jez Moxey said: "All parties need to draw a line under this matter, which has now dragged on for over a month.
"We need to focus on the season ahead and these disruptions to the squad's preparations are unhelpful.
"We have always said we want to keep our best players. If our valuations of them are not met then they will remain at Wolves but we cannot allow this speculation about their futures to drag on indefinitely up to the closure of the transfer window."
Fletcher handed in a transfer request earlier this month, after Sunderland's previous interest was rebuffed.