We use could have + past participle when we want to make a guess about something that happened in the past.In this case, we don't know if what we're saying is true or not true. We're just talking about our opinion of what maybe happened. Why is John late? He could have got stuck in traffic. He could have forgotten that we were meeting today. He could have overslept. We can also choose to use might have + past participle to mean the same thing: He might have got stuck in traffic. He might have forgotten that we were meeting today. He might have overslept.

Shouldn't have + past participle means that something wasn't a good idea, but you did it anyway. I should have studied harder! (= I didn't study very hard and so I failed the exam. I'm sorry about this now.) I should have gone to bed early (= I didn't go to bed early and now I'm tired). I shouldn't have eaten so much cake! (= I did eat a lot of cake and now I don't feel good.) You should have called me when you arrived (= you didn't call me and I was worried. I wish that you had called me). John should have left early, then he wouldn't have missed the plane (= but he didn't leave early and so he did miss the plane). Shouldn't have + past participle means that something wasn't a good idea, but you did it anyway. I should have studied harder! (= I didn't study very hard and so I failed the exam. I'm sorry about this now.) I should have gone to bed early (= I didn't go to bed early and now I'm tired). I shouldn't have eaten so much cake! (= I did eat a lot of cake and now I don't feel good.) You should have called me when you arrived (= you didn't call me and I was worried. I wish that you had called me). John should have left early, then he wouldn't have missed the plane (= but he didn't leave early and so he did miss the plane).