From: Chicago, Illinois
I think the responses on training here are part of the answer, but not the full answer. "National Morale" is really overall military efficiency, which relates to recruit quality, training, doctrine, small unit leadership, and other factors.
I think most would agree that overall efficiency of the Wehrmacht declined from 41 to 45. It declined in recruit quality, training, the loss of experienced small unit leaders....and also declined because of increased concentration of high quality recruits, leaders, and equipment in elite units (SS and others). This had an impact on the "regular" German army.
Soviet recruit quality actually declined as the war went on and the Soviets had to scrape the bottom of the manpower barrel. But Red Army grew in terms of doctrine, small unit leadership, communications, experience, and combined arms tactics. Red Army learned a ton from the defeats of 41-42.
US Army "National Morale" grew for different reasons; recruit quality remained strong, and underlying morale really did not change. What changed was that US Army gained experience, particularly in small-unit leadership, and changed some doctrines to improve.
I can go on with other nationalities, but if you look at it as, instead of "National Morale", but as "Military Efficiency", think it makes more sense