It maybe the following reason:

(1) the student seats of the school or the professors are full, please try to find another;

(2) you have sent email to professors without stating the clear vision of your study plan, research interests and research proposal;

(3) your academic performance (GPA, publications, etc.) or research proposal did not meet the requirements of the school or the professors based on their own criteria;

(4) you have sent group emails to too many professors, which is an impolite behaviour to the professors;

(5) your email might be filtered to the SPAM box, please re-send the email by changing another email box.