How many times have you tried to contact a provider, someone who you know is currently active as she is posting ads and whatever else, sent her a perfectly straightforward request with a day, time, and screening info and not even gotten a reply?
Then you send her a follow up email or two, and suddenly you get lucky and she replies, everything goes smoothly from there?
I understand that it is annoying when someone ignores your email, I really do, but we all get a shitload of emails and depending on when the email arrives, what else is going on, what other emails arrive right before and after, some get lost.
If a reply is important, send a polite follow up, asking if somehow the original email got lost or missed and say that if for some reason she does not want to give a reference, just let you know and you will not bother her again. In my experience, unless the lady is away or flaky as hell, you will get a reply by the second or third polite email. Yes, that may be of no help as the client's desired meeting may have come and gone hours, or days, ago, but it does open up some line of communication.
Do not assume the person who has not replied is an asshole or is doing it intentionally, as that is rarely the case. Assholes are happy to tell you to f**k off.
MF is correct, however in that the client should let the reference know and get her okay first. That way the lady is watching for your request, probably.
zig