For what reason wouldn't gastric bypass work on some people?

Only a tool: Gastric bypass is only a tool to use for weight loss. It works by restricting stomach size and/or decreasing the amount of absorptive surface for nutrition get into the body. You can either eat large enough portions to stretch out the smaller stomach or eat so many calories that even in absorbing a fraction of them you still take in more than you need and gain weight rather than lose weight.

Gastric bypass fail: In order for a gastric bypass to work, the patient must do their part which is to eat healthy food, eat the right portions, and exercise. The operation helps with eating the right portions, but it doesn't dictate what you eat. You can eat junk food, not exercise, and defeat a gastric bypass. Also, overeating can stretch the pouch and make it easier to eat more food.