Teams and leagues are linked to the account you are signed into when creating them.
For example, let's say you have two accounts; FantasyTeam1 and FantasyTeam2. If you are signed into FantasyTeam2 when creating a second team or league, you will not be able to see that team or league when logged into your FantasyTeam1 account.
If this happens, don't panic! You should be able to identify which account is linked to your team. Following the steps in our Yahoo Help article Can’t see your teams or leagues will most likely solve your dilemma, even if you can't remember any of your other Yahoo IDs. We strongly recommend that you do not skim this article.