Is your e-mail going unanswered?

As a general rule the chance of your e-mail getting a response is inversely proportional to the number of people on the To and Cc lines.

Let's take an example:

You e-mail Bob a question. Bob now feels directly responsible for your question and promptly replies.

You e-mail Bob, Anna, and Frank a question and put them all on the To line. Now Bob, Anna, and Frank are only one third responsible each for replying and all three ignore your e-mail hoping one of the other two will respond.

Psychologists call this "diffusion of responsibility".