The German Army was designed around a mobile attack strategy that used flank counterattack as a defensive strategy. They never considered a fighting withdrawal or static defense line and were often surprised to find themselves on the defensive in Russia. read more
As things got bad for the Germans, more and more of the Luftwaffe had to be spent defending the Reich against American and British air raids to the point where 1/2 the German air force was on the defense in Germany and no longer supporting the ground armies in Russia. read more