Posts will not link to CSS

Hello, I’ve been working on this all day and cannot figure out the problem.

I have a category of posts–it will list the most recent post with the stylesheet but all other posts are displaying without any CSS. I was using permalinks for the site (which has other pages) and I switched it from %postname% to the post id number. It preserved the names of the pages, but gave the posts links by their numbers (e.g., /405/) and that did not help.

Are there other things I can try?