pretty much what has been said before really!
Blitzkreig tactics worked offensively but defensively the germans weren't the force they once were.
Hitler decided to attack Russia, and with the invasion of sicily and eventually the defeat of facist italy, and the D day landings, effectively Hitler had a war on 3 fronts.
also later on hitler did not listen to his generals and interfered where he really should not have.
Also a mention of the psychological factors, a war on three fronts and the force of the allies must have been worrying!