Panzer Group Kliest consisted a total of 41,140 motor vehicles with 1,222 of them being tanks. Had this force been lined up on a single road it would have stretched from the border with Luxembourg all the way ton Königsberg in East Prussia. In practice this equated to a marching force that was 400 kilometers long spread across only 4 roads through the Ardennes (Frieser, 110). As you can imagine this cause a tremendous bottleneck in these first days of the campaign.
(Freiser, 117)
By May 12th only Guderian's 1st Panzer had maintained its timetable and had advanced to Bouillon near the French boarder. Guderian's 2nd Panzer was behind that and Reinhardt's units were all still on German soil stuck at the German border. You can imagine what an opportunity this would have been had the Allies known that there was the big log jam of Panzer divisions in the Ardennes. Did the German's really manage to keep reconnaissance flights from flying over this mess? Well, no, they didn't. In fact on the 11th a pilot of a reconnaissance aircraft reported to the French 9th Army that he had seen a long column of German vehicles in the Ardennes. Again on the 12th a reconnaissance plane returned from over the area, riddles with bullet holes and leaking fuel with a pilot reporting a large force in the Ardennes. The intelligence section of the 9th Army refused to believe it (Frieser, 142). Between the 9th Army's disbelief of their own reconnaissance reports and French Air Commander d'Astier de la Vigerie preoccupation with the activity in the north added to the already preconceived notion that the Ardennes was a fortress in itself since it would be impossible to put a large armored force through the thick woods. Today would have been an ideal day to strike a decisive blow on the Sickle Cut Plan while only Guderian's 1st Panzer was in the clear. They could have had a devastating effect on the Panzer Armies and my have completely upset the German war plans.
Freiser, Karl-Heinz. The Blitzkrieg Legend: The 1940 Campaign in the West