You may have quit the app before it was done syncing. Open the app and wait until the sync icon in the upper right stops rotating. The sync may also stop if you do not have a good internet connection. The sync may be triggered manually by tapping the sync icon when it is not rotating. It's best to sync when using a good WIFI connection.

Note: sometimes there are sync problems when on certain WIFI networks. Disable WIFI on your phone and try to sync using mobile data.