I can sure understand how frustrating it is not to receive a response to emails. But in most cases I would bet the problem is not with the sponsor, but with the method of delivery. Emails are great tools, but spam filters reject piles of them everyday, and the recipient has no way of knowing anyone had tried to contact them. As a business owner, nothing is more frustrating than to miss out on a potential opportunity.
PM's here are good to a point, but they are not fail safe either. I receive email notifications of PM's as they come in. A couple of weeks ago I received an email notification that a PM had been delivered. But I have never seen the actual PM come through. PM message boxes are limited in size, and if the receivers box is full, yours won't go through. That wasn't the problem with my missed PM, but could be on others.
Your safest bet is that when you don't receive a response in a timely manner, pick up the phone and call. I'm pretty sure that the sponsor will be very happy that you did.