If duplicates aren't the issue, then chances are that the email address is being used by multiple members. Again, if you have members that need to log into the system, then they must be the only people in the system using that email address. Facebook mandates email address uniqueness, so we think we're reasonably allowed to do the same. It's how we identify who is accessing the system.
To see if this is the issue, go to the People Search and enter the email address of the member who is experiencing problems.
As you can see in the screenshot, the email address is being used by everybody in the family. How is the system to know who it is trying to log in? Basically, we have no way of knowing. It also is not necessary to record the email address against everybody. The original registration takes care of communication options, and parents will automatically receive emails without hard-coding the email address against each player in the family.
Resolution
The way to resolve the issue is to clear out the email address against everybody who is using it except your member who is trying to log in. If it's their email address, they should retain it. This will clear things up for the system as to who is logging in.