Sometimes your device may stop recording or seem unstable. This applies to any device that uses SD cards or other memory cards, including spy cameras, CCTV cameras, some GPS Trackers and of course listening devices.
Remember that virtually all devices require a Class 10 SD card, which is basically how fast it can operate and record. If you are not using a Class 10, then change your card. You may also be using a cheaper card, please use a well known brand as your reliability will improve.
Many times this is due to the units battery going flat or other interruption half way through recording and leaving a corrupted file on the card.
Sometimes it is because your SD card was not formatted from the factory.
To resolve this you will need to remove the card and reformat it. You can find a great article here on how to do that on PC and Mac.
http://www.wikihow.com/Format-an-SD-Card
Formatting your SD cards will fix many problems not normally associated with the problems you may be experiencing, as will using the correct Class and a quality brand.