What’s the contents of your tailwindcss.config file? Make sure you have properly configured where TailwindCSS will search for classes in your project using the content key:
content: [
"./js/**/*.js",
"./css/**/*.*css",
"../lib/*_web/**/*.*ex",
],
Anything outside of these files will not get generated: Detecting classes in source files - Core concepts - Tailwind CSS
