Adam Johnson was given a tour around OT by Fergie and Gill but came back and said he did not want to leave Middlesborough due to his mums health. United made a bid for Bale but Spurs offered more. Ramsay chose Arsenal over United. Wilshere was an Arsenal lad from young and unlikely to move to United since he was already at a top club.
United have a very good scouting network in the UK, but as has been mentioned previously the 90 min rule makes a big difference. There are also limited places so they try to get the best players for the positions they need. They cannot just stockpile players just to stop other clubs getting them. Overall they have a pretty good youth system judging by the number that have made the first team and playing at other clubs in the leagues.