First of all, check the "Loop Rec" setting, you may set to "Off" . change to "On" will do loop recording.

Secondly, SDHC cards are prone to failure over time. Each bit of storage on the devices can only be written-to and read-from so many times before they fail. the memory card usually have extra storage that's invisible to you. When a storage block dies, one of the extra blocks is allocated to take its place and so the total data you can write to the memory card remains the same.

Eventually, when all the extra storage blocks are used up, the memory card will start to lose capacity. Due to the nature of FAT32 format, it won't look like its losing space, but attempting to write data will fail. The easiest solution is to get a new memory card.