If it’s uploaded to Connect, then forget about issues with the device syncing. The problem will be with Garmin Connect and/or Strava.
Maybe try unlinking the connection between Connect and Strava and set it up again. Also check in Connect settings that it lists Strava as a connected service.
You can also try manually uploading to Connect by uploading the FIT file using the Import option on the Activities list, instead of syncing with Bluetooth or Garmin Express. Though as said if it’s uploaded okay I can’t see that will be the issue.
Myself, I find it takes seconds these days once it’s uploaded to Connect. I sync with Strava and Endomndo.
However I have huge problems with Bluetooth sync on my 510. Almost always never pairs with my phone, so I have to connect with a cable. It’s a known problem. If by some miracle it pairs then Connect Mobile will sync but in the latest version it comes up with an error, but it has synced the file, and it then syncs to Strava.
@Travis – being based in China, that sounds like an issue with the Great Firewall of China 😉