Recommend resetting the Bluetooth connection between your Garmin device and your iPhone. These steps and video seems to be the most helpful to fix this issue.
Remove your Garmin from Garmin Connect App (More > Garmin Devices)
"Forget" your Garmin iPhone settings > Bluetooth
Reboot iPhone
Restart your Garmin device
Put your Garmin in Bluetooth pairing mode. Varies by device, so check your manual