Garmin Troubleshooting
Tips and tricks in Garmin
Open Garmin app every now and then
Enable Background App Refresh in IOS settings → General → Background App Refresh
Check battery saver on Garmin watch is not enabled
Check battery saver on IOS is not enabled
https://support.garmin.com/en-GB/?faq=9BcXLSQ4A22gasLarkUvH6
As a last resort remove the pairing of the device and re-add to Garmin connect.