Fitting braces is a lengthy procedure that requires each bracket to be precisely positioned on the tooth. Brackets have to be bonded to an absolutely clean surface, which is why the procedure often has to start with cleaning the teeth. Another factor that affects how firmly a bracket bonds is moisture. If the tooth gets wet before the bracket is placed, the adhesive may not set with its normal strength. Often, when the archwire is put in, such a bracket comes off while the patient is still in the chair. The orthodontist can then deal with it on the spot and bond it back on. At the end of the procedure, I always warn my patients that there is a chance one of the brackets hasn't bonded at full strength. If so, it will come off the first time you eat, even if the food is soft. This shouldn't worry you. Sometimes we simply didn't see that the tooth had become wet. If a bracket comes off very soon after being fitted, contact your orthodontist, who will arrange an appointment to bond it back on.
This answer is for general information. Your own case needs an examination by an orthodontist.