High munition production needs trained personnel to match man with weaponry and convert the outputs in strategic reserves suitable for combat. Otherwise, the additional production is only useful as replacement stocks (i.e. a lost tank can be immediately replaced to the surviving crew).
Training civilians to become tank commanders, pilots, riflemen, radio operators, or the dozens of different specializations necessitated in modern armies, involves more than learning to use their equipment and weapons. They need to learn the tactics that help them to survive and win. A fighter pilot, for example, needs to learn to fly his aircraft with proficiency and then how to operate as a team member in a fighter formation. The same is true in all army trades.
The required skill-set may take as low as a four month of instruction for a rifle man (plus a variable period of advanced training) to as much as 15 months for a fighter pilot .
To make matters more complicated, war conditions change and the training must adapt to it. Survivors acquire experience and they become more lethal against newcomers, so to increase chances of survival of new soldiers, armies analyze enemy tactics and find ways to counteract them. Then, training centers teach these fresh solutions to recruits.

Soviet military training centers proved capable of instructing millions of reservists. By war’s eve, the Soviets had a 4.9 million soldier army with a pool of 14 million trained reservists . In 1938 a Soviet organization, Osoaviakhim, was training 3 million men per year and it had been doing so for more than a decade . This vast number of reservist conferred enormous depth to the Red Army: It could accept grievous losses (285% loss-rate of initially deployed army) and remain an effective force.
The Versailles Treaty, by limiting the size of the German Army to 100.000 soldiers, insured that for a decade the Germans were unable to train reservist causing them to face chronic shortages when they started to expand their army.
The German Armed forces had 3.1 million soldiers for Barbarossa and had another 900.000 effectives in garrisons in the West and Balkans plus only 321.000 replacements . This level of reservists only allowed moderate losses (10% loss-rate of initially deployed force), before the Army started to lose its edge, causing the German army to be brittle.
The level of training in the Red Army proved to be inferior to that of the Wehrmacht for most of the war and this is a prominent factor that explains the favorable kill ratio the Germans enjoyed. This was particularly true in 1941. One reason for that appears to be a more professional stance of the German army rather than a lack of opportunities of the Russians. After all, the Russians trained reservists for a decade and there was ample time to fine-tune training while Germans started later and developed more effective courses. In addition, both Countries faced each other during the Spanish civil war and both fought wars with other Nations during 1939 and 1940, giving them similar experience and opportunity to fine tune doctrines and training.

From carefree youth to recruits to training to seasoned veterans. The industrial war in the East meant that millions of young men from both sides would never return home.

The other reason, explained in more detailed in a later chapter, is the effect of the Stalin purges that started in 1937 and created major dislocation in the Soviet Command structure, some of which had a likely negative impact in the training of recruits.
Regardless, the amount of reserves and training infrastructure allowed the Russians to raise copious quantities of new formations with just enough officers and training to maintain a minimum of fighting capability.
Why did the Germans fail to increase the pool of trained recruits for Barbarossa? Compared with increasing industrial production, the task of training soldiers, although daunting, is less complicated. Training requires weapons, equipment, and vehicles , but they can be of a much more basic design that is easier to manufacture in massive quantities. Skilled instructors are the real bottleneck.
Germany had some significant resources that were untapped: there were thousands of officers, mechanics and specialists in the conquered territories, suitable as instructors for basic training and even for specialized technical training in many cases, to help increase the number of replacements. They remained unused for the most part. A drastic redesign of the training curriculum to streamline it and reduce the number of hours was also possible. This they did, but much later .
Thirdly, German leadership should have directed a huge reorganization of labor since a man cannot be simultaneously in the army and in the factory. Men had to be in the front, therefore others: women, adolescents, foreigners and slave labor, should substitute for them. The Russians carried out a ruthless redirection of labor, increasing the number of women in factories and boosting the daily working hours for all workers.
They used women in the Army in non-fighting roles in 1941 but later, they even used them in certain fighting specializations (snipers, night bombers, fighters and anti-aircraft artillery were the most common).
The Germans could have started this process forcefully after the defeat of France and they chose not to. This was the second critical mistake they made.