Excel Filter not working as expected

I am using Excel 16.31 on OSX 15.1 Catalina

I have a spreadsheet which isn't filtering as I expect. I use filtering on a lot of other spreadsheets, and they work fine, it's just this one.

When I go to filter the sheet, all I get as filters is the first line in the spreadsheet, and nothing beyond that.

I have attached the spreadsheet here in the hope somebody can find the problem for me.

thanks in advance

Charlie