uploading photos to laptop problem iPhone 5

Having problems synching my iphone 5 photos with my laptop - when I plug in, sometimes it 'sees' new photos and uploads them, other times it does not.  I cannot seem to 'force' upload of new pics even by going into the computer, then the iphone and selecting upload.  For no apparent reason, it seems to randomly decide to see new photos and then, the upload goes fine!  Have tried turning both the laptop and the phone off but does not seem to make any difference