I just added a new Tab and a couple of new fields to this new tab but for some reason it only shows up on new user profiles.
By design a field only shows up in a user profile tab only if that specific user has populated this field with some value (via the update profile menu item or via the initial registration process if this fields also appears during registration).
The same is true for tabs. A tab will only appear in a user profile if it contains at least one field with a populated value in it.