Verify that you're running the latest release of Garmin Connect Mobile iOS.
In the Garmin Connect app remove Garmin - More > Garmin Devices > select your Garmin device >RemoveDevice
On your iPhone go to Settings > Bluetooth > select your Garmin device >ForgetThis Device
If you have any other mobile devices (phones, tablets) nearby and paired with the Garmin, also do the same there - Settings > Bluetooth > select your Garmin device > Forget This Device
RestartiPhone
RestartGarmin
Place your Garmin inpairing mode - depends on model, but usually hold MENU > select Pair Phone.
Pair using Garmin Connect,notyour phone's Bluetooth settings.