Permalinks Problem on Post Import

I have a WordPress site being built were I have some pages in the site. I also have some custom post types that contain several posts in each types and my permalinks set to the following:

/%year%/%monthnum%/%postname%/

Everything works well in that posts, pages and the custom post types all display correctly without any issues.

I now want to import some posts from another WordPress site. Therefore I have exported these posts using the built in WordPress export facility.

I then import then in the normal way through the tools menu and all seems fine.

Now the problem. Any page or post I visit shows all the sites posts and not the page. So the following URL – https://domain.com/sample-page is showing like a post archive page. Also the custom post types give a 404 error.

Really weird. I have refreshed permalinks as this has been suggested to be a problem but this has not worked. Any help would be appreciated – thanks in advance.