How to freeze header row in iCloud numbers?

I'm working with Numbers iCloud beta.

I have a few tables on a spreadsheet that I've been working with for a couple of months. Two of them have a frozen header row. One does not.

I can't see any difference in any settings between the tables. I've looked at all the row settings (yes, they all have one designated header row each).

I've clicked every tab in the format panel and every icon in the top toolbar.

Any ideas? Thanks for your help.