One of the failures of the American colonists in this period was the inability to maintain unity in the face of the British enemy.
Although many men would take up arms against the British during the Revolutionary War, it is often overlooked that the enthusiasm for the War among the American colonists noticeably waned as the conflict dragged on. No less a personage than George Washington himself complained about men preferring to stay at home instead of fighting.
Inevitably, as time went on and the initial ardor for fighting wore off, it became harder and harder for the American colonists to maintain the kind of unity necessary to take on and defeat the might of the British Army and its German mercenaries. The authorities tried to get around the problem by offering recruits generous bounties for enlisting.
But this initiative was not enough to attract enough men to the colors, and so it was no surprise when Congress mandated that anyone enlisting in the Army had to sign up for three years or the entire duration of the conflict, whichever came first.