The German 'no' can often come quickly, and seemingly without any consideration. The degree of hardness of that 'no' can vary, however. The most effective way to overcome a German 'no' is to first identify what the reasons are for it, then address them one after the other with convincing counterarguments.
'No' – for now. Germans, too, fear receiving a quick and hard no, from management, business partners and customers. It is not uncommon to receive such a 'no' from a customer, but then be invited to a dinner, during which the discussions address how the parties might at some point collaborate on a first project.
The hard and fast 'no' can become a flexible 'no', potentially even a 'yes'. The German 'no' is seldom a definitive rejection. Instead it indicates that something, some aspect, certain details are not (yet) right, so that a German cannot (yet) commit. No often means "I cannot say yes based on what we have discussed thus far". In such circumstances, Germans will actually use the word Jein, a combination of yes and no, signaling maybe.