Keywords: miss out, train's gone, miss the boat
1. Used in response to someone's request for you to clarify or to repeat something because they have joined or rejoined a conversation you are having with someone else. It is too late for them to follow because they have missed something vital. Idiomatic English = "Miss the boat"
2. You are not really prepared to repeat yourself. It is used particularly when you have just given the punch-line to a joke and someone does not understand because they were not paying full attention.