Hey Javier. Sounds like a frustrating situation for your manager. There are a few common culprits for such an issue.

As AT mentioned, not hitting the submit button will revert the changes. We have a 'Leave the Page?' message to help prevent it, but it still occasionally happens.

Another likely reason is an outdated browser. If the manager is still on Internet Explorer 7, or Firefox 3.6 for instance, many game features will not work. Updating the browser is the quickest way to resolve this issue.

Lastly, some managers will set lineups in advance, then go back and try to add players. Since the player they add is not included in the lineups set in advance, all future rosters are reverted to account for the new player.

The manager either needs to add players first, before making future roster changes, or go through and make them again, after the player has been added.

Hope that helps Javier!