Sometimes if the network connection is interrupted, or the user logs off and back on again, the data sharing system does not synchronise with the remote server, so events are not uploaded (this is evident by looking at the 'Local Data' tab of the Data Sharing page - if it shows some events without remote event IDs next to them, they are stored locally and not uploaded.
Stopping and re-starting the server fixes the issue.
Sometimes if the network connection is interrupted, or the user logs off and back on again, the data sharing system does not synchronise with the remote server, so events are not uploaded (this is evident by looking at the 'Local Data' tab of the Data Sharing page - if it shows some events without remote event IDs next to them, they are stored locally and not uploaded. Stopping and re-starting the server fixes the issue.