Hi
You could try cleaning the jack with some isopropyl alcohol and a Qtip/cotton buds
If you decide to try this don’t pour the alcohol into the jack ,dip the cotton bud in it then clean until it looks clean
If that doesn’t work it could need replacing or repairing
Here’s a guide on disassembling the iPod
[guide|3309|new_window=true]
Once disassembled you will need to ether reflow the solder joints or desolder the Jack depending n if your replacing it or not
Here’s a link to the part you may need
https://www.ebay.co.uk/itm/190748878434?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=R78hLv0QR3m&sssrc=2349624&ssuid=0ACG7idPSQ6&var=&widget_ver=artemis&media=COPY
Hopefully this helps
Any questions please ask
Thanks:-)