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Hello @Millymolly64, thanks for joining the Fitbit Community, it's great to have you on board. 
Have you tried to restart your phone lately? Since when have you experienced this situation? At this moment I would like to suggest you the following steps:
- Restart your Fitbit One:
- Plug your charging cable into your computer.
- Plug your Fitbit One tracker into the charging cable, making sure that the gold contacts on your tracker align with the gold contacts in the inside of the charger.
- Hold down your tracker's button for 10-12 seconds.
- Remove your tracker from the charging cable and press its button until the screen turns on.
- After restarting your Fitbit One, leave it plugged in and charging.
- Go to the Bluetooth settings on your phone and check if your tracker is on the list of paired devices. If it is, tap on it and select Forget this Device. Then, restart your phone.
- Make sure you have a strong Wi-Fi signal
- Make sure there are no other Bluetooth devices that might interfere with the set up process.
- Open the Fitbit app and tap on the Account symbol at the top right corner of the screen, then tap on "Set up a New Device"
- Choose your tracker and follow the onscreen instructions to continue
Please try these steps a couple of times and if you need anything else, please feel free to reply. 
Best Answer
