(I'm at work so this will be short).

One of the main things you need to understand about the Schliefen Plan was that logistics was a really big part of it: Roads, railways, bridges, etc. The idea was that if one man can travel x amount of miles in a day, and if a road can only hold y amount of men marching abreast at a time, some simple arithmetic can be used to determine the best (and fastest) way to move troops from one area of the front to another. Every movement, every reason, every marching order was on a strict timetable to gain as much ground as they could as fast as they could. Schliefen (and his staff) tirelessly ran the numbers again and again but it the plan never worked at the level of efficiency that he desired; there just simply weren't enough roads, railways, and trains to move enough men quickly. So he got it the best he could and set it aside with all the other contingency plans that Germany had.

Then, after Schliefen died and WWI drew closer and closer, Chief of Staff von Moltke pulled the plan back out and made a few changes. Mostly, he added several more divisions to the plan, even though the original number was too many for the Belgian infrastructure to accommodate. Speaking of the Belgians, the Germans did not expect them to put up any significant resistance (emphasis on significant).

So when the plan was put into action, they initially had success but very quickly ran into problems where they had troops backed up for miles behind the front who were unable to move forward because the roads were clogged. That and the fact that the Belgians managed to halt the German advance with their forts at the Battle of Líege (a development that no one expected) stalled them long enough for the French to arrive in force.

Source: John Keegan, "The First World War"

P.S. Sorry, if there are some inaccuracies, I'm at work and unable to double check my source.