I have this USB C coupler to connect 2 USB C cables. The coupler has 2 female connectors:

I use it to extend my cables when charging:

But it doesn’t always work. Sometimes it does, sometimes it doesn’t (e.g., the device is not charging).

So I assumed this coupler was defective, and got another one:

This one has a small LED, and it too wouldn’t always work.

But because of the LED, I quickly figured it out:

If the LED isn’t on (e.g., not current is flowing), I just have to flip one of the male connectors 180°. Then it works.

And that’s what I also have to do with the first coupler I got: if it doesn’t work, flip one of the connectors. I just needed a LED to figure this out