Only files of file formats supported by the Camera Roll can be copied. For details on file formats that can be copied, refer to the instruction manual of the iOS device.
There is not enough free space on the Camera Roll.
Delete unnecessary data in the Camera Roll, and then copy.
Check the following settings.
iOS 5: Settings - Location services, and check whether "PWS Manager" is permitted.
iOS 6: Settings - Privacy - Photos, and check whether access from "PWS Manager" is permitted.