Only two weeks into the operation, on 11 July, the Germans began to suffer logistical difficulties, which slowed the advance. The German Sixth Army was continually delayed by fuel shortages. Eight days later, on 20 July, shortages of fuel were still undermining operations, leaving many units unable to execute their orders. The 23rd Panzer Division and 24th Panzer Division both became stranded during the opening phase. Once again, as it had done during the Norwegian Campaign in April 1940, and "Barbarossa" in 1941, the Luftwaffe's Junkers Ju 52 transport fleet flew in supplies to keep the army going. The situation remained difficult with German troops forced to recover fuel from damaged or abandoned vehicles, and in some cases, leave behind tanks and vehicles with heavy fuel consumption to continue their advance. This undermined the strength of the units, which were forced to leave fighting vehicles behind. Nevertheless, the Luftwaffe flew in 200 tons of fuel per day to keep the army supplied. Despite this impressive performance in keeping the army mobile, Löhr was replaced by the more impetuous and offensive-minded von Richthofen.