If the parent field is required, you need to make sure it is being populated when you create a new record. Just the fact that you have created a filter on existing records does not necessarily mean that the parent id will be populated in the new record.
So you have a few options.
1. Include the ParentId field in your table - forcing the user to put the parentId into the new record. 2. Ensure the ParentId field is one of your conditions. (This should be ParentID, not Parent.ID)
Sometimes I create 2 conditions - on on ParentId and another on Parent.name - in order to ensure that both are available in the new record.