The refund was issued on August 12, and everything looks good on our end as far as I can tell.
Please check with your bank again - if you own multiple credit cards, perhaps it would be prudent to double-check (via a Support ticket) which card was used to make the transaction.
I don't think I've heard of a refund simply not going through, and I've worked Support in GOG since beta :), so my only reasonable guess would be that perhaps you used a different card than you remember, back when you preordered, and therefore maybe you're checking the wrong account balance, and are asking the wrong bank abou the wrong card?
If needed, we can open a support ticket with our payment provider, but those can take a long time to process, so I'd like to rule out the above possibility first. :)