the best thing to do would be to go into your wow options and double check which key is assigned to take them. i mean you might be pressing the wrong 1. or just download and install xfire as this will track your gaming hours and you can take ss and videos with this and upload it to their site so its on your profile. scroll lock+s for ss and scroll lock+v for vids are default.
You might have another program or addon that is grabbing the screen shots instead of WoW. Like EVGA Precision grabs mine and puts them in its own folder.