The Marines were not denied the ammo. As soon as they filled out the appropriate form and checked the “dire immediate need” box, their request would be processed.
My uncle who was a chopper pilot in VN told me the story about when he dropped in on a fire base. They were just getting under attack. Guys had no ammo. Why? Because some idiot in Washington figured that the reason for the high combat deaths was, wait for it, because our guys got into fire fights. They strapped up all the ammo and guys were getting killed trying to get those ammo boxes open. My whole family (all military) despised the Johnson admin. and aren’t too thrilled about what’s happened since.