No matter how strong the referral or relationship, some people will simply not respond. They are too busy, do not think they can help you, do not like to network, or maybe they do not really like the person referring you, etc. There are a myriad of reasons over which you have no control or no knowledge of when you reach out to them.
So be professional and persistent, but if they do not respond after three tries, just move on.
Expectation setting #7: Expect the reject, just don’t take it personally.