The OverPass Chill App needs to be open at all times to collect data from the Chill tags. If your phone restarts by a user action or from a dead battery, the OverPass Chill App does not start up automatically and will need to be restarted.
The other reason for not capturing location data is the Chill Tag may be out of range from the phone.
All temperature data is stored on the Tag and is read when the App is opened or within range of the Tag either in real time or from memory. No Temperature data is ever lost.