This is a weird one, I know. I used a cassette deck that has a square USB output on one end of the cable lead, and a flat USB input on the other end, which was attached to my laptop computer. The cassettes I was transferring were all recorded in stereo. In Audacity, I got two channels from each cassette - but the stereo had somehow been “converged” into two identical mono tracks - no panning of separate instruments like they were on the original cassettes. Everything in Audacity is set to stereo, and yet this happened.
I then tried using the same cassette deck’s RCA outputs (red for right, and white for left), which I brought into the laptop using an adapter that the other end of the RCA cable plugged into. To get it into the laptop, I inserted its STEREO eighth-inch (headphone type) jack, into the stereo input port of the laptop. I got two signals in Audacity, and yet, once again, the stereo was gone, and they were two identical mono signals with no separation of instruments.
I think my gear is haunted. Help?!?!?!?
I inserted its STEREO eighth-inch (headphone type) jack, into the stereo input port of the laptop.
Does your laptop have three different connections for sound? If not, then your laptop may be normal and only have one mono microphone connection.

Some laptops present that one connection as if it were two, but that just results in two-track mono—what you have.
There’s a mechanical way to do a proof. Plug your stereo cable into the laptop and start a recording. Touch first one and then the other of the RCA tips. The other hand should be touching either the rubber cable or the rubber part of the connector. We are intentionally creating a buzz interference condition as a test.
When I do that, I get buzz first on the left and then the right as I change RCAs. I’m guessing you’re not going to get that, so the problem is between that point and Audacity.
Audacity should be set to record stereo.
Audacity > Edit > Devices: Stereo > OK.
It’s also possible that Windows is forcing the system to mono. For that we should wait for a Windows elf.
Koz
No need to wait. We have a FAQ ready to post:
http://manual.audacityteam.org/o/man/faq_recording_how_to_s.html#rinstereo
In the green box in that FAQ, it shows how to check that the USB audio CODEC for the cassette deck is set to stereo in Windows.
Gale