I have same experience. My boy will turn to 3 next month. When he was younger, he was a polite boy. He greeted 看更 every time and he would say hi to others when I asked him to do so. He also would say "唔該晒" loudly to those who helped him. I told him I was proud of his behaviour and this is a good behaviour.
However, in the past 3 months, he refused to do so or only greet people when he was in the mood.
He asked me what's the meaning of "respect" couple days ago, I explained to him with examples and told him that greeting people is kind of respectful behaviour. Today he could do it again. I hope he can understand.
Same experience when my daughter is at 3 years old. However, situation has suddenly changed shen she grows up at 4 years old. Now she is 5 and is willing and actively to say hello to everyone. I don't know it is the common "problem" for such age (i.e. 2-3 years old) children. But I think when they grow up, and the parents always remind them, they will know what is politeness and will know that people like children who are polite.