These things are small (and so are their labels), so it’s easy to mess them up if you’re not paying close attention.
Double-check that you’re using the right headers. These things are small (and so are their labels), so it’s easy to mess them up if you’re not paying close attention. These connectors are also easy to accidentally yank out when you're cleaning up your cables or moving things around inside the case after they've been plugged in.