Until recently I have been buying and stretching my own large as well as small canvases. I always worried about the quality of the gesso, the canvas, and the method of pre stretched pre primed canvases. A friend mentioned to me several problems about the gesso flaking off on pre-stretchered canvases but had no detailed information on particular brands ,companies, or countries making them. I called Golden Paints and talked to one of their technical support persons. She suggested the following articles on problems with gesso. Above are the links  .http://www.goldenpaints.com/technicaldata/AdhesionCanvas.php  and 

http://www.goldenpaints.com/justpaint/jp13article1.php