In Parshat Chayei Sarah there were miracles that happened to Rivka. When she drew the water at the well, the water came up to her. Other miracles that happened to Sarah reoccured when Rivka came to Avraham's camp (the candles burned from Shabbos to Shabbos, the challah stayed fresh from Shabbos to Shabbos, and the Clouds of Glory came to the tent). However, when Rivka went to draw water for Eliezer's camels, the Torah indicates that the miracle did not re-occur, and that Rivka had to draw the water herself. This is because when someone is serving G-d, doing a mitzvah, G-d allows them to do the work themselves in order to deserve and receive a greater reward. We see this also with Noah, who fetched himself only those animals that were going to be used for a sacrifice, and the other animals came miraculously by themselves.