Why does my 16GB SDHC card only give me 20 minutes of video?

I bought a class 10 16GB Kingston memory card for my Nikon S2550, and it gives me 4000 or so photos on the 12MP resolution, but only 20 or so minutes of video. And the lowest video resolution only lets me video an extra 3 minutes, though I've read that it should give me between 120 and 360 minutes of video. I also checked before buying it if it is compatible with my camera, specifically it being class 10, and though I can't remember what I found, what I read assured me that it would work.
Please help? I want it for vlogging.

Added (1). It's a brand new memory card and doesn't have any media on it (in response to the first reply)

Video eats space. If there's already a load of files on that card then the gnomes do not have the space any more to stow bags of video.

Did you format the card before using it
the manual told you to do so
that is one reason

If you read your manual, you will learn that the largest video file your digital camera can save is 2 gb

That tells you that you can shoot up to 8 different 2 gb files on your card