I'm 6 weeks new to the hobby and have met 3 wonderful providers that I've given great reviews to.

What's the polite way to ask for a white list referral in return? With one, it's been less than a week yet (hoping to see her again); with another it's been 6 weeks and I'm likely to never see her again.

Any advice is greatly appreciated - and thanks to the vets for paving the way for an appreciative not-so-newbie!

Just politely ask her for a referral or a white list posting. It helps them too.

Not all providers are going to white list you, as some are not active enough participants here.
As you are checking reviews, also check out the reviewers with white lists. This way you can see which ladies are participating, and visit a few of them.
Be nice, white lists are optional.

Thanks for the feedback. All three were fabulous providers and two are heavily reviewed here. I'll follow up with them!

Why will you never see the one whom you saw six weeks ago? Was there something wrong with her??

If you'd like to get on a white list, ask one or all of them. Better yet, become a member of Date-Check, then if you want to see a provider whom you especially like, you can have her go to Date-Check.
The provider is primarily interested in whether you are a LE, or a nasty guy, or an underage kid, etc.
Don't be timid. If you didn't think a provider was not worth mentioning, don't write a review. If she was 'beyond all expectations', write a review, but if she's got 'pages' of reviews and they are nearly all good, don't bother.