Having been looking for work over an 11 month stretch from June 2011 to May 2012, I have had similar experiences.
At first I would just reply to the job advert and then follow it up after a few days. On very few occasions did I get through to the person mentioned in the advert, usually it was they're on the phone away from their desk, in a meeting, and, yes a message would be passed to them asking to ring me back. Still waiting for a lot to ring me back!
I then started to ring them immediately after I had replied to the advert. This got a better response.
I can only think that as I'm not paying their salary or earning them commission, they don't think I'm important.
One reason I have been given is that the recruiters can not reply to every job application they receive. It is easy enough to compose a station item in email that just needs name job applied for and reason for not being put forward.
On a similar note I have had promises of feedback from (failed) interviews, that I am still waiting for, including from a head of HR for a public sector organisation.
The lack of response from both recruiters and employers I have experienced indicates a lack manners.