What are the chances of gastric sleeve surgery failing in the first couple weeks/months? & how common does it happen?

Sleeve and diet: If by "failing," you mean that your weight did not go down, it is possible. The procedure should be combined with a change in diet. Supervision by a knowledgeable nutritionist helps. The sleeve makes dieting to lose weight easier, but it doesn't make the weight go down by itself. Some people require a second procedure. Should discuss with your doctor.