I have written a spreadsheet using data validation to control input to
certain fields using a dropdown list for the user to pick from. The master
copy is working fine.

I am aware that if data is copy and pasted into the cells the data
validation is ignored and removed from the cells (I believe this may have
happened)

I have taken action to delete the current data in the cells and re-set up
the data validation however the drop down list does not appear. Furthermore
data validation using a dropdown list does not appear to work anywhere else
within the sheet (but can be set up on other sheets within the file).

I can only assume that somewhere within excel there is the ability to
suppress showing drop down data validation lists in a particular sheet (if so
I cannot find the option) or there is an excel error that is preventing a
drop down data validation list from showing.

Could you please advise on how to display the drop down list or obtain if
this is a known excel error. An explanation of how it occurs would be useful
as the user cannot remember what they did!!