How to get people to reply to emails and what to make of a no response?
If you want to know why you get faster responses from grad students than professors, just take a look at this comparison of email volume when transitioning from student to professor.  Afterward the load just continues to rise (the number of emails I get per month is now much greater than anything shown there).
Many researchers -- particularly faculty -- have an email address that is publicly listed on the internet.  That means that anyone in the entire world can contact them about anything at any time.  Some well-known researchers get a huge amount of email, and could not possibly respond to it all even if they did nothing else.  Even a fairly ordinary mid-career professor with some research funds gets a pretty high volume of email from total strangers.  I'm not referring to outright spam but to things like applications for student/postdoc/researcher positions; requests for research assistance, or inquiries about research collaborations and so forth.  Replying to such email cannot trump essential duties like research, student advising, and teaching, so the time for it is limited.
Initial message
Given a queue of tasks (emails) that perhaps cannot ever be completed in the available time, one must prioritize.  If you are emailing a complete stranger, your email is not likely to be at the top of the queue.  The best way to make sure your email stands out is to

Ensure that it does not look like a form letter.  Currently, the only messages I don't reply to at all are those that look like the sender could have sent identical messages to everyone in my department (usually, they did).
Show that you have done your homework.  If you're applying for a job, do you have some research ideas that the professor would be interested in?  What makes you especially qualified for the job?  If you're looking for a collaboration, reference specific things in the contact's papers that are of interest to you.
Along the same lines, don't email a stranger asking them to do (home)work for you (yes, it happens a lot) or requesting information that you could find for yourself on the internet.
Be polite.  Recognize that you are interrupting a complete stranger without having been invited to do so.  You are not entitled to their time; you are requesting it.
Write clearly and concisely.  If it's a first contact and you want to ask questions, try to ask just one question.

Lost messages
It does happen that messages get lost, due to spam filters or by being buried under other newer messages.  If you don't get a reply, there is no way to know whether the message was lost or just didn't make it to the top of the queue.  If it was lost, a reminder may be appreciated by the recipient.
Reminders
Typically, I would wait at least a week before sending a reminder. I feel that a reminder to a total stranger after 2 days is not polite.  In your reminder:

Be extra polite.
Do not blame the recipient.  A good strategy is to say you are sending a follow-up in case your original message went to spam, or something similar.
Remember that you have no idea what is going on in the professional or personal life of the recipient, and you are certainly in no position to  judge their actions.

If you are corresponding with a collaborator, the above rules still apply.  Last month, a friend and collaborator suddenly stopped replying to emails just when we had nearly completed a manuscript.  I waited two weeks, then sent a message just asking if she was okay.  In fact, it turned out that a major personal issue had arisen that -- among other things -- prevented her from doing any work during that time.