Just want to say there is a very simple fix for missing Garmin activities. Since CS get notifications when an activity was added to Garmin, simply downloading the fit file from Garmin Connect, deleting the activity on Garmin Connect, then uploading the fit file to Garmin Connect will get it processed by CS immediately.
I had one activity from 4/26 that was not synced yet (it was still in the Strava queue when I paused that syncing and starting Garmin syncing) and this fixed it quickly.