Format B tables some with a shaded heading and others do not

Try Custom Ruling & Shading: From Table
If they still aren't shaded, a common reason is that the heading row isn't (actually a heading row), but is just a body row to which heading styles have been applied.

Hi Ken:

I also use Rick's TableCleaner plugin because I deal with large numbers of tables, but this post expains how to remove overrides on table at a time:

https://www.rockymountaintraining.com/adobe-framemaker-removing-table-overrides/

~Barb 

There are two possibilities here.

1. The unshaded row at the top is actually a body row pretending to be a header row, and is not actually a header row.

2. The unshaded row at the top is a header row but someone applied custom shading.

To figure out which it is, add a bunch of rows at the bottom of the table to see if the unshaded row carries over.

If the first row carries over to the next page, it's a header row that had a custom shading applied. Highlight the whole row, press Esc-t-x (or click Table > Custom Ruling & Shading), then deselect Custom Ruling Style, Select Custom Cell Shading, then select From Table for both Fill and Color.

If the first row doesn't carry over to the next page, it's really a body row and not a header row. Press Esc-t-a and add a row to the header, then cut and paste the contents of what you thought was a header row into the newly created header row and delete the empty row.

If you have a lot of tables like this, I would suggest purchasing Rick Quatro's TableCleaner plugin, which can, amongst other things, autoconvert body rows to header rows and remove custom ruling and shading from all the tables in a document at once, instead of one by one. It's $39, and I find it to be a lifesaver.