Format Table style with alternating colour not working ("zebra" format)

I have a file with tables and it's impossible to get the alternating fill colours for rows (aka 'zebra' format!) to display properly: odd and even rows have the same fill format.

What i want - zebra

The only format i can get for this particular table is 'non-zebra':

The table format pull down menu only shows formats with non alternating rows (although header and footer row can be of a different format).

For this table the format menu shows only non zebra options:

That file has other tables with zebra formatting ok for which the format menu shows zebra options are well:

I tried a make a custom zebra format, the preview shows ok as below but then the table then displays with same fill format for odd and even rows:

Can you advise how to fix without having to rebuild my whole complex file from scratch?