rows missing in field

[staypuffinpc]

I have a field in File A with consecutive numbers on different lines (e.g.)

1

2

3

4

5

etc.

In a related file, File B, I have created a lookup that appears as a pop-up of this field. I can click on the field and choose a number from the list and that number appears in the field. I then have another file, File C that has a portal to FIle B. This same pop-up field still acts as a pop-up, but there is nothing in the list! What happened and what can I do to get it to work correctly?

Peter