I had the same problem, and for me the cause was a bit silly: I found a tiny round magnetic piece stuck onto the charging contact of the left Nothing Ear (2) earbud. I have no idea if this piece belongs to the earbud, the powerbank, or if it just came from somewhere else and randomly got stuck there without me noticing.
It had shifted and ended up sticking to the charging contact, blocking the left earbud from charging. Once I removed it, the earbud started charging and pairing normally.
If one of your earbud isn’t charging/connecting, check that no small magnetic piece is covering the charging contact