One of the most likely reasons is that they don’t have a child in your school.
When parents sign up, their account isn’t attached to any school in particular, after all, it’s the children that actually attend the school!
During the lifetime of the parents and children at the school they may, at the end of the period, choose to ‘delete’ the child from the school and that’s the moment when they’ll disappear from your list of parents. They’ll still exist in reports and exports but if you need to see them back in the list of parents or, in the list of credits/debits/payments, we’ll need to ‘undelete’ them.
To undelete a parent, contact us and we’ll make that happen.