What version of FileMaker are you using?
If this is FMP 12, try this simple test for the problem portal. Enter layout mode and drag the portal. Does the field move with the portal? If not, adjust the postion of the field so that it is fully within the portal.
Also, make sure that the portal does not touch or cross the boundary line between the header and the body layout parts.
Also keep in mind that the record you are looking at in preview mode is not always the record you were looking at in browse mode. Preview mode previews all the records in your found set starting with the first record in the set. If you are on the third record when you entered preview mode, the differences in data in the first record when compared to the third may be why the portal is empty.
If that doesn't resolve it. Uploading a screen shot of your layout captured while it is in layout mode, may be helpful in understanding what issues might need to be checked.
Filemaker 12, I think I figured it out. The first field in the portal was also set up as a button, and apparently it does not like that there is a button in a sliding portal, because when I deleted and replaced the field it worked fine, then when I set it up as a button again the problem resumed. Thre is only one record, and the field is fully within the portal which does not touch any boundary lines.
I tested out my theory and any field I set to a button shows up blank in a sliding portal, then when the button is set to do nothing or if the portal is set so it does not slide/resize the field works again.
This sounds like a new FileMaker 12 bug to add to the growing list of the same. I encourage you to copy the link to this thread to your browser and then click the Report an Issue tab at the top of this screen. When you document the bug, you can save typing by including a link to this thread in your post. Report an Issue is the section that filemaker techs most often check for reports of possible bugs.
However, I've been testing this with FMP 12 windows and I can't reproduce the behavior you report.
What I did:
I define two tables and just enough fields to have a serial number field in the parent table, a foreign key and one text field in the child table.
I created a related record so that it appears in the portal.
I returned to layout mode and specified slide up | Resize enclosing part. I used button setup... to make this text field a button. I previewed the layout.
I saw a single row portal with the text I had typed in the field showing in the portal. A fill color that I'd specified for the button, however, did not appear so had I had a blank field, the button would not have been visible.
Removing "allow creation..." in the relationship did not have any effect on this.
Perhaps I did something differently. Perhaps you have a mac and this is a Mac only issue.
Note: once I cleared the "Delineate..." setting in layout setup, the field's fill color appeared when in preview mode.
I have Windows, Filemaker 12 Advanced. The portal also has a filter if that might make a difference. I will report in the bugs section though.
What is the filter expression?
If you remove the filter expression, does it make a difference?
Is "Allow creation of records via this relationship" enabled for the portal's table occurrence?
The filter expression is:
If ( Activity::ShowNew = "Show New Records Only" ;If (ProspEx Daily Tickets In Range::DateCreated ≥ Activity::ActivityStart and ProspEx Daily Tickets In Range::DateCreated ≤ Activity::ActivityEnd ; 1 ; 0 ) ; 1 )
The filter itself seems to work fine, as the rest of the fields in the record show up. Removing the filter and sort options does not affect the problem. Records cannot be created or deleted by this relationship, or through the portal itself. There is no scroll bar, but that setting had no effect either.