Hi there,
I have exactly the same problem. Images (JPEGs) are available in the folder (DCIM-CAMERA), videos should be in the same folder but aren't. They appear and work fine on the phone (Samsung Galaxy S5). Also, if I take a video whilst connected to the computer, the video appears in the folder on the computer and plays perfectly! If I disconnect the phone and then re-connect, the video is gone!!
I've checked the file location on the phone which just confirms that I am looking the correct folder. The appearance of the video while connected just proves this further.
I have tried this on two different HP laptops, both about 3 months old and both running Windows 8.1. I have also tried another Galaxy S5 (on both laptops) with the exact same problem (incl. the video test whilst connected).
I have also tried using a Galaxy Note 3 but this works ok-ish. Videos do disappear sometimes but mostly, I can see them/play them ok.
This is indeed very, very strange! If anyone has any clue to what the problem is or has a work-around, I would love to hear from them. I really need to download a video from my S5.