This is coolbert:
That Schlieffen plan as implemented by the German General Staff in 1914 four reasons as generally thought being responsible for failure
1. The size of the German army when mobilized and deployed into the field was too small to accomplish the mission.
As envisioned by von Schlieffen the plan required a German army to be 50 % larger than it was in 1914. The Kaiser opposed such an increase too many additional officer billets not desirable from the standpoint of the old-school Prussian nobility.
2. The neutrality of Holland was not violated.
That German right wing on the Western Front not violating the neutrality of Holland, the sweep and march of the German troops toward the English channel and WEST of Paris not possible.
3. Russian mobilization far quicker than anticipated.
Russian troops entering the fray in considerable numbers far earlier [two weeks] than the German General Staff have deemed as possible.
Forces on the Western Front or those designated for deployment in France rapidly sent in the opposite direction [toward Prussia],. prudent response to the comparative fast Russian advance required.
4. Too much expected from the common German soldier.
Weeks of marching and fighting just beyond the capacity of mere mortals. Even for Germans! Those general officers having formulating a plan "on paper" do-able but not from a human standpoint!
Those assumptions valid in 1905 [that year the Schlieffen Plan adopted] not so valid in 1914, dynamic change during the intervening years have created circumstances not taken into account or even thought inconsequential orignally.