If you have lots of files, especially long source videos - it can take a long time to sync to the cloud. I made a trip video with multiple LONG clips that were edited down to 1-4 second chunks in a montage. ALL of the footage has to sync, and this can take a while. Make sure you have a good connection, no timeouts, and grab a snack.
If you want to see what's sync'd on your cloud, check out https://assets.adobe.com/files on a browser - you can manage your files there as well.