Filtered search results do not show all the correct results

Hi – I want to allow site visitors to search by multiple UK nations – England, Scotland, Wales, NI.

In ‘place settings’ have added a field for nations and for ‘option values’ I have put: {optgroup}|England/England, Scotland/Scotland, Wales/Wales, Northern Ireland/Northern Ireland{/optgroup}

I have put the settings for it to show in ‘advanced search’ as a checkbox filter. However, when I search by nation only those place entries checked as England show up whereas those place entries which are checked as Scotland etc are not returned in the search results.

Not sure whether I have added the ‘option values’ correctly (even though I am able to check the boxes for the relevant nation when I am adding or editing a place entry).