Running activity not syncing to Garmin Connect. Activity is shown on FR645. Other activities do sync.

Probably a corrupt activity (fit file). Try a manual upload of your activity to Garmin Connect

How Do I Manually Upload an Activity I Recorded to Garmin Connect?

If you get any message other than Import Complete, then there is a problem with the file. It is likely corrupt. Attach it to this thread. We can take a look to see if it is repairable.

• Zip the file, then in a reply post Insert >> Image/video/file >> click Upload >> browse for file >> OK.
• Or upload the fit file to a file sharing service, such as OneDrive, Google Drive, Dropbox, etc., make the file public, then paste a link to the file in your post.