Answer by  
                                                                                          Sett (1838)
                                                            
                          
                          
                                                          Weak shoulders can be caused by a number of things such as a joint injury, or an inflamed ratator cuff which is when a tendon gets inflamed or thickened. Weak shoulders can also be due to something as small as using the wrong pillow which can cause tremendous back and shoulder pain. A doctor can do tests to find out.