There are several potential reasons why your Garmin activity might not be uploading to Strava. Here are some common issues and troubleshooting steps:

1. Connectivity Issues:
- Bluetooth Connection: Ensure your Garmin device is properly connected to your smartphone via Bluetooth.
- Internet Connection: Make sure your smartphone has a stable internet connection (Wi-Fi or mobile data).

2. Garmin Connect App Issues:
- App Update: Ensure you have the latest version of the Garmin Connect app installed.
- App Permissions: Check if the Garmin Connect app has the necessary permissions to access your phone's data and internet.

3. Account Linking:
- Account Sync: Verify that your Garmin Connect account is properly linked to your Strava account. You can check this by going to the "Connected Apps" section in the Garmin Connect app or website and ensuring Strava is connected.
- Re-link Accounts: Sometimes unlinking and then re-linking the accounts can resolve synchronization issues.

4. Activity Sync Issues:
- Manual Sync: Try manually syncing your Garmin device with the Garmin Connect app.
- Data Upload: Check if the activity data has been uploaded to Garmin Connect. If it hasn't been uploaded there, it won't be pushed to Strava.

5. Strava App Issues:
- App Update: Ensure you have the latest version of the Strava app installed.
- App Permissions: Make sure the Strava app has the necessary permissions to receive data from Garmin Connect.

6. File Errors:
- Corrupted Activity File: Occasionally, the activity file might get corrupted. Check if the activity appears correctly in Garmin Connect. If not, it might not be able to transfer to Strava.

7. Temporary Outages:
- Service Outages: Sometimes, either Garmin Connect or Strava might be experiencing temporary outages or maintenance. Check their official websites or social media pages for any announcements.

Steps to Troubleshoot

1. Check Connectivity: Ensure your device and phone are connected and have internet access.
2. Verify Account Link: In Garmin Connect, go to Settings > Connected Apps and ensure Strava is listed and linked.
3. Force Sync: In Garmin Connect, pull down on the screen to manually sync your activity.
4. Check Uploads: Open Garmin Connect and ensure your recent activity is listed there.
5. Re-link Accounts:
- Unlink Strava from Garmin Connect.
- Log out from both apps.
- Log back into Garmin Connect and re-link Strava.
6. Manual Upload: If the automatic sync fails, you can manually upload the activity file:
- Export the activity file from Garmin Connect.
- Upload the file to Strava via the Strava website.

By following these steps, you should be able to identify and resolve the issue preventing your Garmin activity from uploading to Strava. If the problem persists, you might want to contact Garmin or Strava support for further assistance.

You know, I bet the number one reason is a lot of new Garmin bike computer and watch users join Garmin Connect (advised to do so by the packaging) and mistakenly assume Strava and Garmin are the same entity. They wrongly think data would be shared between the two without any extra intervention, or that their Garmin device uploads directly to Strava. In reality, these are two seperate corporations and an authenticated connection must be established for each user/member. While this is a one-time thing and not difficult to do, the end user has to be the one to okay it because of privacy issues. I'll point you to this help article for the details:
https://support.strava.com/hc/en-us/articles/216918057-Garmin-and-Strava
That being said, there are a few other reasons. Heavy usage times like Saturday afternoon in Europe or North America might cause a slight delay.  Wait and look again after a few minutes! Or cyber terrorists might cause a denial of service attack. This actually happened to Garmin in 2020.