Yes, it's probably the surname that's tripping you up. To create a new profile, only two fields are required: First Name and Last Name at Birth. (You can enter Unknown for either when necessary.)
When you added your mother, it probably auto-filled (i.e. guessed) at the Current Last Name (married name), e.g. Small. But it would have left the Last Name at Birth (maiden name) blank. A yellow message at the top of the page should have alerted you that it was missing.