Supervisor in German Universities
A "cold" request sent to a faculty member—sent when there is either no announced opening for a position or no established cycle for admissions—is unlikely to generate a response. This is because of the sheer amount of email that German professors can receive, particularly if they run large research groups. (In many cases, the public email address may not be directly handled by the professor but be filtered by an administrative assistant.) In that case, there's not much you can do.

Your best bet is to look for groups that have announcements for open positions and apply to those. In such cases, you may get a response, but even then there is a possibility you might not hear anything back. It's basically saying "no" without sending a rejection email.