1) The Schlieffen Plan was too complex and over-relied on the speed and precision of the German army. The German army was unable to move as quickly as anticipated and the French were able to slow their advance significantly.
2) The French army was stronger than anticipated and was able to mount a strong defense. The French were able to hold their ground against the German onslaught and even launch successful counterattacks.
3) The British Expeditionary Force (BEF) arrived in France earlier than expected and played a significant role in the Battle of the Marne. The BEF helped to shore up the French lines and prevent a German breakthrough.
4) The Russian army launched an offensive in East Prussia, which forced Germany to divert some of its troops from the Western Front. This weakened the German offensive against France.
5) The German supply lines were stretched thin and the German army was unable to keep up with the demand for food, ammunition, and other supplies. This caused morale to suffer and made it difficult for the Germans to continue their offensive.
As a result of these factors, the German plan to knock France out of the war failed and the war settled into a stalemate on the Western Front that would last for several years.