iv just read something on Dunkirk and it was talking about how the nazis could have potential decimated 400,000 British soldiers. so why dident they. this goes in hand with other things iv heard like how in d day no one woke Hitler up to deploy his panzer divisions. and even once we was awake he waited until the evening to do anything. why would they not use every advantage available to them?
While we cannot know what Hitler and the OKW thought when they ordered the German armoured formations to halt, there are several facts that can help explain why.
The German armoured formations had advanced very far in a very short time. They had outrun their foot-marching infantry support and were potentially vurnurable to a counterattack. Today we know that no strong counterattack or any breakthrough attempt came from the Anglo-French forces, but the fact that the armoured divisions and the infantry divisions were far apart worried the German high command.
The terrain around Dunkirk is boggy and criss-crossed by rivers. It is very bad tank terrain. The British also had a reputation among the German high command to fight very hard when their backs were against a wall (which was an experience taken from the fighting against the BEF 1914 and the Spring-Summer offensives 1918).
Göring claimed that his Luftwaffe would be able to stop any evacuation of the British troops from the sea. The only time a large army had been evacuated from sea in modern times had been at Gallipoli, and that had been ~135 000 men in a well-planned operation from several prepared points with the Ottomans having no air force to interdict the evacuation from.
The German armoured formations were worn, on several cases spread out, in need of supply, rest and refit. While they had cut off or destroyed most of the best of the Allied forces, a majority of the French army still awaited behind them. The armoured divisions were needed for the upcoming attack against France, Fall Rot as they were the best formations of the Heer. Risking them at Dunkirk, and perhaps wearing them out further, giving the French time to re-organise their defences probably seemed like a bad idea at the time.
The bottom line is that the Germans worried about the exposed positions of their armoured formations, did not believe the British would be able to evacuate under German air attack, did not want to use armour in bad terrain, worried about a potential breakthrough attemtp or relief attempt and wanted to spare their best formations for the upcoming finish off of France.