Under their Parents, press What are you trying to fix? and choose "A parent is missing". A gap in the roots? You can pick someone already on the tree, add someone new, or keep a space with an empty card until you know more. For example, if your mum's dad isn't there, add him as new, or choose him from the tree if he's already on it somewhere.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com