Many people have been complaining that the device will no longer record after it gets full. If you bought your card on Amazon or e-bay, there is a good chance you have a fake card. I could get into the details but there is a great you-tube clip from a professional tech reviewer - see attached videos.
If you purchase directly from Amazon, and not a 3rd party seller on Amazon, you are 99% safe from fake anything.
Fleabay is a different story, lots of crap going on there.