The most common cause of a 'weak' chin is a 'trapping' of the lower jaw when facial development occurs. This occurs as a result of the upper jaw not developing fully which leads to almost all malocclusions including overbites, underbites and crowded teeth. This underdevelopment can be caused by several factors. Chronic mouth breathing or having a tongue tie are two of the most common reasons for having a low tongue posture which typically is the cause of upper jaw underdevelopment. The lack of ability to have the tongue at the roof of the mouth when at rest is the culprit. Low tongue posture causes a lack of stimulation needed for the normal development of the upper jaw. Then the trapping can hold back the lower jaw and results in the recessed lower jaw, do mostly a displacement posterior of the lower jaw. It is a bit complicated as you can see.The correction involves freeing up of the lower jaw to come more forward. the jaw joints have to be evaluated to understand how far the lower jaw can come forward, but usually this can be achieved non-surgically, but the dentist/orthodontist needs to fully understand this principle. My book Straight Talk about Crooked Teeth explains this in more detail. Good luck with your treatment.