I don’t think that not being absolved at a particular time means that the sin can never be forgiven. It may be that because of the circumstances the sin couldn’t be forgiven at that time.
Remember that when a person goes to confession they have to be sorry for their sins and they also need to have a firm purpose of amendment. In other words, they intend not to commit that sin again.
Now imagine that someone confesses that they’re living with/having sex with a significant other. OK, they’ve confessed their sin. But if they’re going to go home and continue living with the other person, that doesn’t demonstrate an intention not to continue committing the sin. Therefore it can’t be forgiven.
If later on the couple splits up and the person goes to confession again, now the situation is different and the priest could give absolution.
There are probably better examples, but that’s what comes to mind at the moment.