@kugiigi Thanks. I've investigated a bit further. I'm using a Qi charger not an NFC charger (although NFC charging technology does exist apparently) so the NFC being enabled or disabled wasn't affecting the wireless charging of the Volla phone at all.
It looks like it was just a coincidence that charging by USB started working after I switched off NFC andd rebooted
Ive tried out a few combinations of things and everything is working correctly.
I'm going to put the USB charging problem down to the battery being down to 4%. I've seen another comment about the Volla phone acting strangely at times below 10% charge. Possibly switching off NFC reduced the load on the low battery enough to get USB charging working again. Or it was all just a glitch.
Worth raising the issue though in case a pattern starts to emerge with other Volla users.