Curious if anyone has any ideas of what I can do about this:

I had a fully-flexible one way ticket (booked via Etihad.com) for last Saturday that I had to change to this Saturday for reasons. Last Friday I phoned the Etihad call centre, made the change with the agent, received a payment link, made the payment and got a receipt by email from Etihad (but, no new ticket which I didn't notice for same reasons). Today, I try to manage booking and it no longer exists. Phone call centre and they say the change was not done because the change fee payment is marked as a "soft payment" and hasn't been received by Etihad from the credit card company, so I was marked as a no-show. My only option was to pay the no-show fee plus a new fare difference (or buy a completely new tickeT). I pointed out to the agent that I had a receipt for payment, from Etihad, which by definition implies that they received the money. But no go.

Other than raising a complaint which will, I'm fairly certain, be completely pointless. Is there anything I personally did wrong here other than not notice that the new ticket had not been issued?