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.
The adidas Nitrocharge is the most talked about new cleat in years. It was released on May 15 and will take your pitch by storm.
adidas has added some color to the Predator line with this new colorway. Released on May 17. Be the first to own a pair.
Bayern Munich are kings of Germany and hope to be crowned kings of Europe at Wembley later this month!
The new PUMA evoSPEED 1.2 FG. A great new soccer cleat with classic look and first time for PUMA to be on the cleat.
The Juventus jersey for the 2013/14 was released earlier this week. Be the first to wear the colors of the Old Lady.
The Liverpool 2013/14 soccer jersey is available for pre-order. The Reds are eager to compete for silver next season.
Borussia Dortmund hears Wembley Calling and are headed to London for the Champions League final.
One of the top soccer cleats for control the Nike CTR360 will not let you down on the field in tight spaces.
Chelsea won the Europa League on Wednesday. The Blues defeated Benfica 2-1 on injury time goal from Ivanovic.
The push for more color on their cleats, adidas has added the infrared colorway to the adiZero f50 Line.