Cassette Deck only playing one channel

Ever had this issue? I’m recording cassettes to Audacity and it’s only been playing one channel. I cleaned the heads and checked the connections. It’s an older Optimus cassette deck.

Windows 10

Looks like stereo-pair, but both tracks are the left channel ? …

How about the analog outputs?

How are you connected to the computer?

Do you have a desktop/tower computer with a “regular soundcard”, or a laptop?

What’s your operating system?

I am going from a cassette deck to the back of my desktop computer using the line in. It records fine other than one track is only playing from the cassette deck. Only the left channel levels show any output. I’ve used lots of alcohol with swabs. I’ve ran the cassette head cleaner a few times. I know alcohol/Swab method is recommended. I’ve used a known stereo store bought cassette to test it.
I use Windows 10.
Another problem I just started having is with the volume on my computer. When I turn it up, it has a high rotating hissing sound coming through the speakers. I turned the level down in my sound controls and that helped, but it’s not nearly as loud now. I used to have it on around 90% with no problems. Now I can’t adjust it to over about 50% without getting the hiss. I’m wondering if it is my sound card.
I’ve only had this HP desktop for about 6 months, bought new at Best Buy.
Thanks for any ideas.

Are you 100% certain that the cassette deck is putting out a stereo signal?
Can you connect for example, amplified PC speakers directly to the deck’s outputs and check for audio on both channels?

If there is audio, then move on to check the sound card.
Connect the headphone output from your mobile phone to your sound card’s line in.
Does it record in stereo?

I plugged my cell phone’s headphone out to the computer’s line in. It did record in stereo. Must just be my tape deck.