There was no Prussian Army in WW1, Germany was unified in 1871. One might argue that it's semantics since the Germany Empire was about 2/3's Prussia... anyway.

Why a campaign in unsuccessful or successful is often a gravely overlooked field of military history. Mostly because attributing success to individuals who give orders in a battle is much easier and more in line with our tradition of glorifying war to various degrees. That part is comparatively easy.

It's easy and interesting to analyze how the orders and movements in a battle affected the outcome. It is much more difficult to analyze why those specific units were on hand. What the logistics and supply situation was like. What the morale and combat effectiveness of individual units was like and so on. The more you look at it, the more complex it becomes.

I would say that there is no single identifyable cause that caused both Prussian armies and Third Reich armies to have unsuccessful military campaigns. There may be common factors that contributed to campaigns failing in both scenarios, depletion of experienced officers and veteran soldiers for example, but you'd have to look at individual campaigns for that and they wouldn't be unique to Prussian and the third Reich. You would find the exact same factors affected everyone from Alexander the Great to Napolen at some point.