The Black Cats have reportedly made a new bid of around £12million for the striker.
The club have previously had two bids rejected as Wolves want around £15million for Fletcher.
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."