Cannot change date-time format in cells of Apple Numbers

If you try to change the format of a cell when the contents of the cell cannot be formatted that way, the new format will not take. For instance, you cannot format the text "abc" as a "number". Numbers is probably not recognizing your "date" as a date and is instead treating it as text. You may have to create a temporary new column to change things around in the "date" to put it into a format that Numbers recognizes then copy/Paste Formula results back to the original column. Or you may be able to temporarily set the spreadsheet to a different region, one that uses that date format, format it as a date there, then change the region back.

You never posted an example of a date that won't convert so it is not possible to either agree or disagree with your statements or to provide any additional help.