What can cause a person not to lose weight after bariatric surgery?

Compliance. Weight loss surgery is an assistive device, if you will, for weight loss. It doesn't force the weight off the body. Patient compliance is a big contributor to weight loss, and not following a good diet and exercise regimen after surgery will hinder weight loss despite the operation.
Takes time. It takes upto 9 months to see the results of weight loss surgery. The reason for not loosing weight after the initial improvement is slowly building the amount of food intake - the stomach can regrow to fit the increased food portions. So if it is initial after surgery be patient if it is a late effect you might need to add other protocols to complement the surgical benefits.
Type of OR & diet. The type of surgery can influence weight loss. Bypass > sleeve > band. Diet is also critical. For all procedures, eat solid, dense foods, not sliders that go right through. Don't drink with food, since it flushes food down. Avoid sweets. Avoid high calorie snacks like chips, cheese, nuts, peanut butter, crackers. Go to followup appointments.