Well, it's taken a bit but after receiving a message from
@Calligal I finally figured out what the problem was ... for some unknown reason,
@Calligal Lisa went onto the email distribution list for List 5 even though her name was on List 4. And
@Mrssamiali Sami went onto List 2 & 4 distribution lists even though she was only on List 2.
So unfortunately, Sami sent exchanges to TWO lists but only received back from one. And Lisa sent all of her exchanges - but to the list she received in her email (List 5). She received from List 4.
My apologies to everyone as this was not either of their fault. I have no idea how it happened because the email distribution lists are automatically generated from a mail merge. However, this is why it is important to check your list when you receive it and make sure your name is on the list.