I have a Sharepoint list switched in to the modern version but the look and feel didn't change although it says its already in the "new experience". It still displays the classic format. I've tried switching several different sharepoint lists under the same site and they worked perfectly fine. Does anyone know what could be the reason? Thanks in advance.
Have you checked the list page? Please edit the page of the list with issue, if there is a classic web part like a script editor, it will make the list unable to switch to modern experience.
Here is a blog having more information, you can have a check.