Hey there!
There are two possibilities that I know of for this issue. The first is the temperature of environment. In colder temperatures, the device loses the ability to maintain power. Be sure to keep the device in a room temperature environment to ensure longest possible power life. Are you in a cold enviroment while using the device? The other is that the battery life time is limited. The battery only has a limited amount of life within it. Therefore, if the battery is fully charged and will still not sustain the charge then the battery is most likely worn out and requires replacement.
Hope this helps! :)