Same problem. I have a Galaxy S5 mini and after 2 months of flawless performance I lost the Bluetooth connection.
- I uninstalled the app, powered down the phone, switched it on again, re-installed the app (historic data still there).
- Next I went into the Bluetooth settings on my phone. I unticked the "only visible to paired devices" box and hit SCAN - it found the Vivoactive HR device and asked for a PIN number.
- I then went into the Bluetooth settings on the watch (as described above) and selected "Pair mobile device" - a 6 digit PIN number appeared on the screen which I typed into the phone and miraculously the phone now recognises the watch.
- I manually synced the watch and was prompted to allow alerts from Garmin device. All data now up to date.
Thanks everyone - the above all helped to get me to a solution.