I've had this issue many times and finally contacted a Samsung rep. She said it was a driver issue and suggested that I un-install the phone drivers and get the computer to scan for new devices. Yet this only resulted in the driver installation throwing up errors at me. She then suggested tinkering around in my computer registry and that did nothing to fix it.
Finally, she told me to simply restart both my phone and computer and THAT seemed to resolve the issue. It may seem too simple to work but it worked for me so it's worth a shot.