Mourning Dove won't fly away?

Are you sure the Mourning Dove is a female? It might be a male and inside your garage or somewhere around it, it has a female mate watching over their nest. If the Dove tries to attack, this could be the case. Or, this might have been someone's pet, (some people do have mourning Doves as pets), and he/she probably misses his/her home and your garage probably reminds him of his old home. Check it's feet. If it has a band, (a ring around one of it's legs), then it is somebody's pet. If so, then try posting ads on the internet saying you have a Mourning Dove in your yard that won't leave and if anyone has lost one, to please contact you. Try to coax it into your hand with some Dove food or wild bird seed. If it is tame and it comes to you, then it is/was most likely someone's pet.