This sign usually is because your child did not understand the direction or just forgot. It likely does not have much to do with defiance or intentionally not cooperating. Be sure he heard the instruction or has read the direction. Have him repeat it to you and tell you how he will follow it. This lets you know he got the instruction and understood it.
You say, "Change your clothes after school." When he tells you how he will do it, some detail is important. When he gets home, what are the steps? He goes to his room. He takes off his school clothes. He gets out his after- school clothes and puts his school clothes where they belong. He then gets dressed and goes onto his next activity. Most any instruction can be put into a series of steps to follow. Also, most of the earlier discussion on forgetting works here too.