There were several problems. It was unpopular war since Pres Johnson never adequately explained to the American public why the war was necessary. There was dissention in the ranks as many (white) students took advantage of student deferments to not serve, leading to charges this was a war fought by blacks and poor whites who were drafted. The other problem was the US Army wanted to fight a conventional was similar to WW2 and Korea, and the NVA and VC used different tactics that frustrated the army generals like Westmoreland who was unable and/ or unwilling to adapt..,
