In order for you to bring back a team from an old TeamRaiser for use with a new TeamRaiser, the team captain of that team must be registered for the new TeamRaiser. If you see that the team captain is registered for the TeamRaiser, make sure it is the same constituent record. If the team captain has registered but used a different email address and was not logged-in, it will not properly associate the registration with the correct record (constituent IDs must match).