When you find a POW mining you get included at the back of the witness-miner list, then climb to the top spot and then you should witness a block creation and get a reward. This moves you out of the witness-miner queue and you start mining again in order to get included once more in the list. It should normally take between 1 and 2 hours to move to the top spot in the list.

Sometimes when your turn comes you may not successfully do your job as a witness in the short timeframe you are being given and you get not reward, should not happen very often, but can still happen.