@killjoy
Rough! Saying sorry was (and continues to be) a huge struggle for my eldest, but I think the truth is some kids just aren't emotionally mature enough by 5 to grasp the importance of it. We can force them to go through the ritual, but if they aren't ready it's a hollow gesture. Expressing disappointment, and explaining to them what the proper behaviour is is probably enough IMO.
Sometimes I find I'm forcing my kid to say sorry just to perform "good parent" in front of others rather than because it's actually beneficial.