Have you compared the free space on your Nexus 7 after you move videos to the tablet verses after they are no longer visible. If the free space has not changed then I would suggest ES File Explorer and make sure the show "." option is enabled in settings. There are multiple ways of hiding files and folders in Android like creating a zero length file named, ".nomedia" , in the folder that you wished to not have indexed or renaming a folder to have a "." before the folder name.