Why they don't even give me a reply?

Tweet

I am a Chinese graduate student who will graduate next year. I am intended to further my doctoral study abroad, to be more specific, in Germany. But now I encounter a problem that most professors I tried to contacted even didn't sending me a single reply, no matter accepte me or not. I am so confused! Was I not polite enough? I checked those e-mail I have sent and found it is not the case. In fact, I have managed to be polite. I think they should at least send me a letter just out of politeness. Maybe they are too busy to check my e-mail!. There was one with whom I exchanged some letters several times, but after I sent my information in detal to him I could never hear from him. I wonder why! Did you encounter the same problem? Who can give me some advices? I wll be extremely graful!