Here I am confused about the standing point from where we look at the past events. Is 'playing' or 'snowing' which we should focus to look the past in the sentence "After we (were playing / had been playing) for an hour, it started snowing"? Which one in the choice should be selected?
I am not a teacher.
I'm not entirely sure that I understand your question, but here goes.
'After we had been playing for an hour, it started snowing", is a correct sentence. The 'playing' was taking place before another past event started, in this case the 'snowing'.