I was watching the first episode of Greatest Events of WWII In Colour on Netflix. At about 22 minutes in the narrator explains the Allies have positioned their forces in Belgium to fight the Germans. Meanwhile, intelligence reports through scouting planes that the German's are planning an attack in France through the Ardennes, however according to historian James Holland (the show credited him as the author of The War in the West) in this show, the French refused to believe it was credible. The show doesn't go into any reason why the intelligence would be believed to be untrue, however. Do we know why this information would have been unbelievable and ignored?
I'm going to give you a rather short answer, because I don't have a lot of books to support my point, I'd rather tell you what I've been taught in class when I was younger (in France).
There were two main reasons on why it was unbelievable :
the first one is that nobody thought that the Third Reich would ever attack the Kingdom of Belgium which was (like the Kingdom of the Netherlands) considering themselves to be neutral. Even if wars are pretty brutal, there is always a notion of code of conduct on what you can or can't do. Attacking a neutral country that doesn't want to choose a side (for whatever reasons, can be political, strategic, economical) is non-sense.
The other reason is that most of the French government by that time was made by WWI veterans. And, to be fair, they weren't very bright. At least in the way that they were stuck on their view from the 1910'. They thought the Nazis would never go through Belgium, and the Maginot line was the best line of defense since it worked before.
In other words, the French were pretty stuck in the past and thought the Germans would follow the rules and the same path as they did previously.