Well there could be a few reasons for this.
When you plug the phone into the computer, are you trying to access the files via a specific program? Or when the popup shows that you have connected the device (if it does) what option are you choosing to view the files?
I am wondering because, in the case of the first option, the program may not read the device correctly. In the case of the second option, if you are choosing anything other than to just view as a folder, the files may not be viewable.
When you plug in the phone, and use the folder only option, it should show you two choices, the main device storage and/or the card.