I'd say about 80-85% of providers do write back, but there are definitely some who aren't what we call "reference friendly." It probably has nothing to do with you (unless, they've looked you up and have decided that you're *too* new - meaning, that you're not yet established enough for them to determine that you're legit. Getting more reviews might help, if that's the case. Until then, consider including a link to your ad on a major forum (Eros? P411?), so that they know you're currently advertising).
Depending on your market, some providers can be very competitive and catty. Others are just not responsible about checking email, etc. I've gotten zero replies to reference requests, I've also gotten replies, like, 2-3 months later.
One thing I might add to your reference request email, is to say that any info shared with you will remain confidential... it wouldn't make a difference to me personally, but it certainly can't hurt to include it.
As for your other question - NO, it is very smart to double-check Whitelist referrals! Personally, most of the clients to whom I've extended Whitelist referrals have specifically requested that I do so. I'll feel compelled to do it if the client is *safe* (if I refuse, I'll make an enemy and possibly garner a revenge review)... but, the last two guys I've WL'd are actually on my do-not-see-again list. If someone PM's me directly, I might be inclined to share more info. Like, 'Well, he was definitely safe and reliable, but I had XX and XX issue with him. YMMV.'
The same thing applies to p411 OK's. As has recently been posted here, p411 makes it very hard for a girl to refuse to give a referral (even in the case of shorting the fee). Always double-check, you might be glad you did. And even if you uncover nothing but 'Yes, I saw him and he's fantastic', then you'll feel reassured.