Why would a revision's surgery not work as well as new surgery?

My doctor said I may not lose as much as I want because my surgery is a revision of an old gastroplasty. Why would this be so? He said there has not been sufficient research done in the matter. I've also read the same information. Any ideas from anyone out there?

We greatly appreciate your interest in helping us build our Q&A database. To discourage vandals from posting garbage, however, we require people to register before posting.