Here's a tiny social conundrum. When someone writes you, calls you and leave a message, calls and doesn't, or even texts you, what is your obligation to respond? If the question is "are you free for lunch today" and you know you are not, is it OK to just not answer? If the question is "have you seen this?."
This seems to come up, and different people and different communities seem to have different rules about this. So before you take offense at the lack of response, consider that it might just be a different social protocol that you may not necessarily adhere to!
I've had that happen, though, when leaving several emails or support tickets with services that I use. What do you make of a lack of response? Sometimes I imagine that they are so close to going out of business that responding to emails has become a low priority. Then on the other hand, I am the email@example.com, and do I always answer all emails that come in? Almost all, but I have to admit that occasionally I've just left an email gone unanswered.
There's this idea of "lifetime value of a customer". If I could predict the future and how much this customer will eventually spend on my business, I can compare that to the cost of servicing them and decide that actually they are going to lose me money and essentially "fire the customer." Of course there are the 'externalities' to such a decision, like developing the reputation of being a business that's hard to live with.