Thank you - You bring up a great point. If you don't know someone and have not interacted in the past, it is very hard to figure out why they may not be responding. It could be many things - they don't respond to unsolicited communication, they don't prioritize it, its not their preferred communications channel, it didn't speak to their interests, they are flooded with other work, etc. In that case I think you should reach out to them a few times in different channels, with some time in between, but if they then do not then respond you should assume they are not interested. Also, one mistake I see all the time is formal pitches from people I don't know. A much more effective strategy would be to establish some kind of casual relationship with me first by commenting on my blog, reaching out on Twitter, etc. I am *much* more likely to pay attention to something sent to me by someone I've already had some contact with - and I think that is universally true.
So, proceed with reaching out but if over time you are not getting a response, it's time to move on in my opinion. There is no hard and fast rule but the more one sided communications you send out without a response, the higher the likelihood you are pestering someone