Is it your first time setting up the device?
If it is, it seems that you might have a pairing issue. Check out the troubleshooting steps under "WHY CAN'T I PAIR MY FLASH TO MY PHONE?"
Already using it for a while and now it will not sync?
If your device is running Android 6.0, please turn on Location service and try again.
Otherwise, please try the troubleshooting steps below 3-5 times. We highly recommend that you try these when phone's battery level is higher than 30%:
1. Press Flash. If you do not see any light on your tracker, or if the two hands do not move on your smartwatch, please try replacing battery following the instructions in the video below.
2. Take the battery out and reinsert it in your device. In case you have not changed your battery for 2-3 months, please try a new battery.
3. Keep the unit close to phone and try syncing again.
As the last resort, reboot your phone and repeat the steps above.