Updated (8/1/17):

We are aware of an issue that causes sync problems for some Garmin devices in older builds of the Windows 10 Creators Update.  Impacted devices include the vívoactive®, vívosmart®, Forerunner® and fēnix® family devices.  The issue is resolved in Windows 10 build 15063.447 and higher.

 
To install the latest updates, go to Settings> Update & security > Windows Update > Check for updates and install any available updates until your device reports that it is up to date.  You can confirm which OS build number you are running by going to Settings > System > About >More info (if needed).