WordPress not auto-inserting paragraphs in posts (but only for specific theme)

Hello all. I’m making my own theme for my new site, but when I post a test post any line breaks are reduced to a single br tag instead of a p tag irrespective of how many times I hit return. However, this is only happening when the site uses my custom theme, whenever the site uses the default page the posts show correctly.

For example, when using the default theme, this post:

“test

foo”

WordPress will correctly add in a p tag between “test” and “foo”. As soon as I switch to my new theme it adds in a br tag instead of the p tag.

Any ideas on how I can fix this?