I've had similar experiences with this - some clients don't read emails. At first, it was annoying frustrating. Although this may not be directly applicable to you, I've ended up managing feedback on a client-to-client basis.
For example, I've got one client who pretty much leaves his mail client open all day. He always responds in a timely manner, and really understands the benefits and speed that come with email. Obviously, this works out well for both of us.
On the other hand, I have another client that is a small business and this business is run by people that are of a slightly older generation. Their business never required the internet, so email never became a critical point of their business model and, as such, don't see email as the valuable resource that it is. To that end, I make it a point to schedule phone meetings with them. It's how they do business and how they believe good business to be done.
Above all else, what I had to learn was this: Despite the fact that email is convenient to me, it may not be convenient or used as often by the customers with whom I deal. At the end of the day, I'm developing projects for the customers. I want to have the reputation of someone that is easy to work with, and that genuinely cares about the people providing me work. If I fault them for not doing business the way that I want to do business, then I'm doing something wrong.
As soon as I got passed that, I felt much better about conducting business. Sure, certain projects may take longer to develop based on the mode of communication, but, at the end of the day, that doesn't really amount to anything significant. Not only do I have happy customers that come back for additional work, but word-of-mouth generates additional opportunities for me.
I don't know how well this answers your question, but I thought I'd sound off on my experiences with this issue.
EDIT: After reading rizzle's answer, I do want to add that making graphical mockups (when applicable) saves an exponential amount of time when, say, developing a UI or website for someone. They can see something that makes sense to them. Writing code, getting feedback, then having to spend hours changing something in the code based on the feedback is so frustrating - mockups can help mitigate a lot of this.