Seems to happen quite a bit.
I sent one a while back and after 2 weeks I get a pissy response stating he had been on vacation.
I then asked why the hell he had an ad if he knew he was going on vaca.
Got another smart ass response.
The ones that do not respond are either stupid, a scammer, or just don't give a shit.
Personally, if I do not get a response within 24 hours, I go elsewhere, not worth the stress.