You said you were sorry, but the other person is still upset. It is often not a lack of sincerity. The other person needed something besides the apology.
The apology does not say what it is for. Sorry alone does not show whether the person apologizing understands what went wrong, so it sounds like a way to end the situation quickly.
The other person may not have wanted an apology at all. If they wanted to hear why it happened or what will be done differently next time, an apology alone resolves nothing.
It seems to them that there is nothing more they can do. They admitted the mistake and apologized, and when nothing changes, they do not know what else is expected.
It also starts to seem unfair. When the other person keeps bringing it up after the apology, they feel they are being punished, and next time they are reluctant to apologize at all.
They see apologies differently. For someone who sees an apology as admitting a mistake, the apology is the end. For someone who sees it as the start of making things right between them, what comes next matters more.
They also need different amounts of time to recover. Some are fine right after an apology, and others need several days to settle. The second group is not being petty. They simply take longer to process it.
Say exactly what you did wrong. Saying you are sorry for forgetting the plan and making them wait tells the other person their feelings were understood. Sorry alone does not tell them that.
Say what you will do next. Adding a sentence about what you will do from now on turns the apology into a promise. Keep the explanation of why short so it does not turn into an excuse.
The apology becomes a formality. Apologizing for the same thing every time makes the words hard to believe, and the other person concludes that nothing will change even after an apology.
The other person stops bringing things up. If they believe that raising a problem only brings the same apology, they stop raising it and instead quietly decide about the relationship itself.
When an apology does not work, it is often not a matter of sincerity but of the two recovering in different ways. What a person needs in order to settle differs from person to person.
The six steps to repair in the relationship analysis show an order suited to the two people, from checking in on feelings right after an argument to a short look back a few days later. That order shows what to do after the apology.
This page describes behavior patterns for self-understanding. It is not a medical or psychological diagnosis, and it does not replace professional care. If difficulties persist and disrupt daily life, please seek professional help.
This page covers the type in general. How it actually shows up in your own combination is what the measurement tells you.