Hi MsDeeC,
Thank you for posting your query in our community.
Per your description, you have problems about the show/hide field in SharePoint list. However, I tested with the same formula on my side, there was no problem. You can check below images:
When I select other options, the field doesn't show:
When I select "Sales, Marketing & Advertising", the field shows:
Given this situation, could you please provide a full screenshot of your formula for me? And if you apply this formula to another list, will it work?
Best regards,
Jazlyn