There are two reasons this happens:
The one that happens most often is that the system either wants a date (birth, marriage, or death), and won't let you proceed without a date. For living people, you don't need a date, but you do have to check the button for "still living" under the death date field.
The other is that you edited your own profile while you were entering your mother's (switching between tabs). The system does not like that, and will make you reenter your mother's profile.
Let us know what happens when you try to enter her again.