Those “missing” relatives could be hiding under a first name of which you are not aware. I have two great-aunts (one by birth and one by marriage) had actual first names that they never used past their childhood. They used their middle names as their “legal” name. That middle name is on all their documents, their tombstone, etc.
Except in records before they were married–those all list them by their “real” first name.
Is someone hiding in records because they have another name of which you are not aware?