Keegan argues that the fundamental failure was that it didn't allow enough forces to be concentrated in eastern France in time for the decisive battle.

The first version of the plan (from the mid-1890s) called for the Germans to destroy the fortress line on the French border before advancing into France to engage the bulk of the French army. Over the next few years, he gave up on the viability of quickly destroying the fort line, and shifted the centre north over several drafts of the plan until the whole German army was deployed through Belgium. This meant the army was constricted to a front of roughly 300 kilometres. With parallel roads spaced roughly 1-2 km apart, this put a practical limit of around 30 corps that could reach the vicinity of Paris within the 6 weeks allotted to the plan.

The plan also depended on destroying any British force deployed to Belgium or northern France, but made no allowance in the timetable to do this. It further acknowledges the need to defend the lines of supply and occupy the cities of Belgium and France, but again doesn't allocate any soldiers.

When he then comes to the actual battle in front of Paris, he realizes the need for a further eight corps (200,000 men) on the far right flank but doesn't explain which roads they have taken to get there. In a later amendment, he writes “On such an extended line we shall still need greater forces than we have so far estimated.”

John Keegan, The First World War (1998)

The biggest problem was a lack of flexibility. The plan was detailed to individual train schedules, troop densities and scheduled lines of advance. The lack of flexibility meant that once war began the German General Staff couldn't mobilize against just Russia (although France and Russia had mutual defense pacts at the time only Russia was at war with Germany when the mobilization order was given, but the plan couldn't be altered to reflect that - to answer an imminent invasion by Russia France must be invaded), or avoid invading Belgium despite diplomatic realities. The timeline was unalterable without throwing the entire mobilization into chaos.

Another problem was the German General Staff thoroughly underestimated the diplomatic reaction that their invasion of Belgium would cause. Many German military leaders believed that the Belgians would not contest the invasion and would allow the German Army to pass through unmolested, and that Britain wouldn't support Belgian neutrality by going to war when presented with a fait accompli of a total German victory in the west.

In reality, the Belgians fought back and slowed the advance, the British declared war and the BEF was key in the Battle of the Marne which ended the battle of movement (i.e. the final point of failure of the Schlieffen Plan) and began the static warfare which characterized the war on the Western Front for the remainder of the war.

So, the Schlieffen Plan was predicated on several assumptions and could not be altered or adjusted in the event. If any of these assumptions proved incorrect (in 1914 essentially all were incorrect) it risked the plan as a whole.

Dan Carlin's Blueprint for Armageddon claims that fatigue in the German Army prevented them from fighting effectively, is there any truth to this?