Newbie Question...Why Won't This Field Show Up?

[oceantracks]

I have a several layouts which display a number field which is an auto entered serial number.

When I made an invoice layout, I included this same field, and it correctly displays the number.

But when I hit "PREVIEW" to look at how it will appear when printed, no numbers show up in the field. This only happens when in PREVIEW.

What am I doing wrong here?

Thanks so much

Tom