Welcome to Android Central! The card could be corrupt or counterfeit. Do other cards work?
Insert the malfunctioning card into your computer and run chkdsk to look for bad sectors: http://forums.androidcentral.com/am...guide-using-chkdsk-fix-corrupted-sd-card.html. You can also insert it into the phone and run SD Insight to make sure it's what it says it is. A counterfeit card might claim to be higher capacity than it really is. If the card already is close to its true capacity, then it won't be able to save anything else, even though the system thinks there's plenty more space left.
Also, make sure you didn't tell the camera app (assuming we're talking about Android), to save videos to an existing folder on the card. The app has to create the folder or the device has to be rooted. One app can't write to another app's folder.