My neighbour's female gsd is more territorial than my male gsd. So its not as issue.
Yes she may of shy nature or was taken away from her mother too early.
Try to find out what makes her bark...a loud noise, a cat, a new dog or somethng else. If u can find out a cause then u can teach her to bark when u want her to bark. Praise her if she barks at something & say Bark/Speak. Repeat this everytime she barks at something. Now say Bark/Speak randomly & praise her a lot if she barks. Gradually she will associate the word with action. Then command her to bark if the bell rings.
Be patient & consistant. Dont forget to praise her everytime she barks at ur command. She should learn.