Make sure your connected phone is with you the next time you drive and that Bluetooth is on. Open the app before you start your trip. The app will not relaunch if it’s been killed/closed, your phone has been restarted, you’ve recently updated your OS, etc. If the app isn't open, it won't connect to the adapter and won't be able to sync when your trip is complete.
See this FAQ for additional help with syncing a trip.
See this FAQ for an explanation of the different syncing statuses.
Trips that have not synced yet will be stored on the adapter and will appear in the app the next time you sync. See this FAQ for more about stored trip data.