I would begin by making sure that none of your other devices that are paired with the iPod have their Bluetooth turned on and that no other device that you've paired the earbuds to is on. In other words, make the connection exclusive; turn off the Bluetooth on everything else that might interfere with this setup.
If that doesn't fix the issue I suggest that you unpair the iPod and the Sony WF-1000XM3 and then pair them (again, making sure that nothing else is on at the time).
Like most Bluetooth devices, these earbuds can be paired with several devices at a time but can only play music from one device at a time, so the steps above are designed to rule out the possibility that other devices are trying to grab the connection.
If that still doesn't sort out the problem, can I suggest that you try the Sony support site as there should be more focused help there. The link (if Apple leave it up) is https://www.sony.co.uk/electronics/support/wireless-headphones-bluetooth-headphones/wf-1000xm3/articles/00274970
If the link gets removed, simply go to Sony's website, lookup the earbuds and from that go to the Support or Questions & Answers pages which will be linked at the bottom of the page. Curiously, I notice that Sony refer to the device as headphones, not earbuds (not that it makes any difference).