The Fitbit app on certain phones with certain devices can sometimes fail to sync. This might happen a couple times a month. To workaround this issue, try force stopping the Fitbit app. This normally shouldn’t lose any data, and reopening the Fitbit app and dragging down should re-sync in a few seconds.
Also, check that Bluetooth is enabled on the Android device.
Force stop Android app
Force stop an Android app by:
Settings → Apps and Notification → Sell all apps → scroll to app name and click → click Force Stop