No response from potential PhD faculty
Doctoral admissions in the United States is usually done at the departmental level. Departments convene admissions committees and these usually utilize one of the following strategies for selection:


Candidates are chosen who are presumed will work with one particular faculty member (or a small group of faculty) who will serve as their advisors or project leaders. 
Candidates are chosen without faculty assignments and are expected to find an advisor / project group in the first one or two years


This is very discipline and department/university specific.

In the case of type #1, yes it helps to contact a POI (professor of interest) who will be your presumed advisor.  In the case of type #2, there's no real advantage to contacting faculty. 

However, you should note that even in the case of #1-type departments, faculty do not respond to cold-call e-mails for all sorts of reasons, so one cannot read too much into non-communication.  Also, in many #1-type departments, the decision is still ultimately made by the admission committee and not by the individual faculty member, which is why they may tell you that the decision is out of their hands.

I suggest writing to graduate students in the department whose interests best align with your own. They can give a sense of which type of selection committee the department utilizes, and which faculty generally respond positively and promptly to e-mail.