Unable to record stereo input in garageband

I have a Mac Mini (late 2014) that I've been using to record my record collection onto iTunes using GarageBand. Lately I've discovered that the audio input port is dropping one of the stereo channels from my amp. I've eliminated the phonograph stylus, amp and cables as possible problems, and there is a straight cable run from the amp's tape-out plugs to the MAC - no couplers or adapters. I even replaced the existing cable with a new one, but the problem remains.

Thinking I might have a problem with the Mini's line-in port, I acquired a UCA202 audio interface and hooked it up to a USB port, connected my cables from my amp tape-out ports to the UCA202, started a recording using Garageband - and I got exactly the same results: only one channel records. I double-checked by connecting the headphones output from my tape deck in place of my amp, and got the same results there too.

I have a Y-adapter which I can use to put both stereo signals onto the one channel, and that works as far as hearing all the content, but I should be able to record in stereo, shouldn't I?

I downloaded Audacity to my Windows10 PC (a Dell Optiplex 3050) and tried to record using the UCA202 - same symptoms: missing one channel. So, I have the identical problem on two different PCs running different operating systems and using two different pieces of software. Worse, on the MAC, I have the problem using the line-in port as well as the UCA202.

As a final test, the UCA202 has an earphone jack, so I plugged in a set of earphones and switched on the monitor to see if I could hear both channels in the headset - and I can.