Hi,
What is Bluetooth driver you use?
BlackBerry requires to turn BT option on manualy before if can receive something. Also, how do you send file? If you do it with wclOPPClient then by default it has Authentication turned off, so you should set an Authentication property to True. Also you have to use wclAuthenticator to provide default PIN on PC side. If you use wclProximitySender then you should use wclAuthenticator as well.
Regarding drivers, not all drivers can send to many devices at the same time so such drivers will process send operation one-by-one.