"Yes" in the German context is more exception than rule. Germans are reluctant to enter into an agreement without being sure that they can deliver. They respond almost instinctively with a "no", a "maybe" or with reasons why they cannot (yet) enter into the agreement.
Seldom will Germans respond with an immediate "yes". For a "yes" in the German context has a very high degree of binding character. Far more than a statement of intention, the German "yes" is the equivalent of giving their word, of entering into an oral contract, something not done without having given the agreement serious consideration.
A "yes" in the American context is more rule than exception. Americans almost instinctively say "yes" to assisting a colleague, often without reflecting on whether they have the time and resources to deliver.
However, the American "yes" can signal different degrees of binding character. The instinctive "yes" means: "We are colleagues. In principle I want to help you."
The degree to which the "yes" is binding, or reliable, depends on many contextual factors. And these are clarified by questions about time, resources, interest, and other obligations.
The spontaneous American "Yes!" does not appear credible to Germans. Americans seem to want to say yes to everything, without first thinking through if they can deliver on their promises. What Germans call American over-promising can become a serious problem in transatlantic cooperation.
It should not be a surprise that Americans expect, and therefore miss, getting a yes from their German colleagues. Americans sense immediately their reluctance. It can appear that Germans are not helpful, not team-players. When Germans respond that they need to first check out the details, Americans suspect it to be an excuse.
Advice to Germans
Beware of American over-promising. It's not a sign of unreliability, but of spirit. Gain clarity about the binding character of that "yes" by asking the w-questions: who? why? by when?, and of course, how? Flush out how serious a well-intended „yes“ is. Get concrete.
At the same time, listen carefully to the conditions. They could signal a „polite no“. The more conditional the „yes“, the harder the „no“ being communicated. When in doubt, simply explain that your command of nuances in the English language is limited, and that you are not sure whether you are hearing a „yes“ or a „no“.
Advice to Americans
A German „yes“ will not come quickly, but when it does, you can rely on it. In contrast, brace yourself for the German „no“. It will come often, and you will perceive it to be harsh and uncooperative. It is neither. It is sober and respectful. Don‘t be deterred.
To determine its level of binding character, inquire as to the reasons why the agreement cannot be entered into. Identify the barriers and overcome them one by one, with questions, suggestions, reasons. That once monolithic German „no“ can be converted into a good, solid, reliable German „yes“.