Moving mp3 files from iTunes to iPhone 14 - synch does nothing

Hello, I have a number of songs I collected over the years in mp3 format which I have been able to add to my iTunes library. The songs have not been purchased from Apple Music but were saved from purchased CDs. I've connected my iPhone 14 to my PC with a cable as usual and I attempted to synch it to iTunes. Nothing happened this time, although I had done this many times before (although years ago). I've tried everything - automatic synch checkbox (nothing happens after I click Apply), manual management (dragging a song from Library to the device doesn't do anything) and I'm now out of any ideas. Can anyone help?