New source folders won't compile

 In project settings, when I add a content root, then mark a directory in it as "Sources", it does not compile into the "Output path" specified in the "Paths" tab. I have done this successfully with a different content root in the same way, as far as I can remember. Moreover, if I mark it as "Tests" instead, it successfully compiles into the "Test output path" specified in the "Paths" tab. Furthermore, if it is marked as "Sources", then I perform "Invalidate cache / Restart", the content root is still in the project structure, but it is no longer marked as "Sources". Does anyone know what may be going on and how to solve it?