Three weaknesses of the Continental Army were:
Little organization: Until the creation of the Continental Army, the Miltias were mostly a ragtag bunch that got ready "in a minute's notice" (as they were called minutemen). This hampered the organization of the army, and led to strategic deficencies as well as disorganization until the arrival of George Washington as leader and the intervention of France.
lack of equipment: Because of their dependence on Britian for arms and rounds, the restriction of trade made it hard for Americans to resupply, as most of their cities created only certain products, leading them to rely on allies for help.
untrained: While most militiamen were able to fire their weapons, they were not trained for the art of war, and generally fought in a disorganized fashion.
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The answer is not:
lack of purpose, because the US colonies had a purpose. They saw how oppressive the British government was to them, and they wanted, at first, a end to the rising taxes, and when that fail, their independence. Without a purpose, the army wouldn't have a spirit of fighting, and would have lost
hope this helps