Mac Excel - auto increment of date not working on one sheet

Hello,

I am trying to drag a a custom date format (ddd dd/mm/yyyy) down a column range, i.e. I highlight two cells with incremented dates and then I drag this down to get (what I expect) to be sequential dates....but I don't get this. Instead I get a repeat of the highlighted cells and the year increments but not the day.

On a new sheet within the same workbook it works absolutely fine, it's only on this other sheet where I have this issue.

Does anyone have any ideas as to what setting needs to be altered to correct this one workbook sheet issue?

Thanks.

A