I know nothing about Netflix beyond the basic concept (I'm not a big DVD watching person, and the ones I do want, I buy, so don't have any rental service), but I'd have thought the Netflix are simply managing the overall demand on their titles in the most efficient way.
If there's a very long wait on certain titles, and you have other titles you also want to see that are available, then it makes sense to send you the other titles, and send the in-demand title to people who JUST want to see those in-demand movies. It strikes me as the most efficient way to manage the overall demand burden on their system, given that (presumably) everybody pays basically the same monthly fee and the resources (ie the number of DVDs in the system) are finite. Yes, they could license more copies, but that would be a terribly inefficient way to manage spikes in demand. As long as the vast majority of customers are happy, and receiving DVDs, then they will grow at a rate that will outpace losing a very small minority who dislike the demand management protocol they're using.
Alternatively, it's possible there's just a glitch in the system.
Call them. Even if there's no glitch, the call will doubtless re-designate your prioirity to a higher level, changing the formula they're using to decide who gets what when. Squeaky wheel, and all that.