It appears that I am having the same issue as many of your users. I thought that I would post my experience in the hope that it will help you diagnose the problem.
Windows 8.1
Garmin 310XT, Software 4.50
After having the issue that my 310XT syncs with GE - but the activities do not show up in Garmin Connect I completely uninstalled Garmin Express and went through the following steps.
Step 1: Installed Garmin Express 3.2.29
* Uninstall current version of GE
* Restart computer
* Install GE 3.2.29
* Restart computer
* 310XT syncs with Garmin Express (3.2.29)
* Activities show up in Garmin Connect
* Delete All History on 310XT (in case one of the activities was corrupt)
Step 2: Update Garmin Express to 4.0.13
* Update GE version using program updater
* Restart computer
* Create a dummy activity
* 310XT syncs with Garmin Express (4.0.13)
* Dummy activity does not show up in Garmin Connect
* Follow the following instructions
- Quit Garmin Express
- Press the Windows Key + R
- Type %programdata%
- Click OK
- Open the Garmin folder
- Open the CoreService folder
- Open the Devices folder
- Open the Unit ID folder of your device
- Delete the Sync folder [note: only thing in this folder was CompletedUploadsV2.xml]
* Delete All History on 310XT (in case one of the activities was corrupt)
* Create a dummy activity
* 310XT syncs with Garmin Express (4.0.13)
* Dummy activity does not show up in Garmin Connect
Step 3: Uninstall Garmin Express (4.0.13) - Reinstall GE 3.2.29
* Uninstall Garmin Express 4.0.13
* Restart computer
* Install GE 3.2.29
* Restart computer
* Create a dummy activity
* 310XT syncs with Garmin Express (3.2.29)
* Dummy activity shows up in Garmin Connect
* Delete All History on 310XT (in case one of the activities was corrupt)
Step 4: Uninstall Garmin Express (3.2.29) - Reinstall GE 4.0.13
* Uninstall Garmin Express 3.2.29
* Restart computer
* Install GE 4.0.13
* Restart computer
* Create a dummy activity
* 310XT syncs with Garmin Express (4.0.13)
* Dummy activity does not show up in Garmin Connect
* Delete All History on 310XT (in case one of the activities was corrupt)
I have tried to be thorough and systematic in this process. In my case at least, I don't think that the problem lies with the Sync folder. Every time that I have checked this folder it only contains the "CompletedUploadsV2.xml" file. According to what has been written on this forum, the fact that this folder is empty after a sync event means that the activities have been sent to Garmin Connect and Express deletes the activity files.
When I look at my device in Garmin Connect, it will show the last Sync event that correlates with the last sync with version 3.2.29. When I have a subsequent sync in version 4.0.13, it is not recognized in Connect.
I would like to be part of the solution. If there is a trial that you would like for me attempt, please let me know. You also have my permission to look into my Garmin Connect account to see if you can find something there.