During the earliest stages of the war, Washington's army had to focus mainly on survival and gathering the necessary supplies and trying to train and learn how to fight as an army. The British were the dominant military force in the world and standing up to them in open battle was almost always resulting in losses for the Colonial army. This led to the reliance on some bold strokes and luck in actions like the crossing of the Delaware.
As the war progressed and particularly after the French began to support the Colonial efforts, Washington's army grew in their ability to compete with the British openly and they had the time and money necessary to complete more vigorous training, etc. The ability of the Americans to move away from the ocean, because they were tied to coastal supply lines, prevented the British from chasing them because British supply lines were always tied to ports and the ocean.