Why did the peace settlement with Germany prove so difficult to enforce in the years after 1918?
In terms of reparations, or the $40 odd billion that Germany was required to pay to the Allies, you can't get blood from a stone, as they say. Germany simply did not have the money, and when France confiscated payments in the form of Germany's gold reserves, it sent the German economy into a deeper tailspin, which in turn made it more difficult for Germany to pay. As this happened, France and Britain became less likely to hound or force Germany to pay because they simply didn't think it was possible. Also, reparations were more important to the Allies when the Treaty was negotiated than it was in the 1920s when they were to be collected.
Because World War I was so utterly horrible, bloody and destructive, a wave of pacifism swept through other European countries in the 1920s, and these countries actively sought ways to avoid future wars (Kellogg-Briand Pact, Washington Naval Conference), and were in no mood to enforce harsh terms at that point.
You would think that it would have been pretty easy to enforce this. And maybe it would have been except for the fact that the British and the French were not really all that willing to enforce it. They put the treaty in place and just hoped that it would work because they did not want to have to go to war to enforce it.
The thing with treaties is that the only real way to enforce them is to go fight the people who don't obey them. So, for example, when Hitler later stopped letting coal from the Saar leave Germany, for example, or when he started to increase the size of the German armed forces, England and France had a choice--they could let him do it or they could go to war.
The problem was that they were not ready or willing to go to war to enforce the treaty. Because of that, the treaty was difficult to enforce.