Validation drop down arrow list disappeared

Hello,

I use a spreadsheet as a calendar with a separate sheet for each month. I
have the days going across the top and a list of staff names down the side.

In the middle of each sheet are drop down menus (using cell validation)
where the staff can select if they are "In" or "Out" (plus a load of other
options).

This works fine on every month except one where there is no drop down arrow
when you click in the cell.

I have check the cell validation and it says the same as all the others and
am pulling my hair out!!

Is there a sheet option somewhere that I may have missed that restricts cell
validation or anything else that prevents this from showing?

Many thanks.