My experience is that each lady definitely has their own rules on whitelisting a gent:
Some girls don't do it at all. Period. For any guy.
Some girls do it automatically after you've seen them once.
Some will only do it if you ask them.
Some will only do it if you review them (and presumably give them a good review).
Some will only do it if you see them multiple times (I had one lady that wanted to see me a second time before whitelisting me - I asked and she said it was just her policy, nothing to do with me personally).
But I wouldn't take anything personally (especially regarding whitelists) because it seems the rules are all over the board. And ladies are very busy. And they forget. Which is something I never do. Forget that is. Over years of honing my mind to the razor sharp instrument of whit and whimsy it is I never forget a thing. Like those girls do... Now what was I talking about again? Uh - no I didn't forget! My train of thought got momentarily hijacked - it's a totally different thing!!! Oh yes, whitelists.
Anyway, if it's a lady that gives whitelists and if she said she would be ok with providing you one then I'd wait a couple of weeks after you've seen her and then I'd politely remind her - once. If she still doesn't do it after that then you can assume she just doesn't want to do it. Whether the reason she doesn't give one at that point was something personal (like not giving her a 10/10 review) or not you likely will never know, I'd just move on.
Do you need more whitelists? Not really. Some would argue that you don't need any at all as long as you have a couple of reliable, recent references. Actual references seem to be the common requirement for any new lady to see you. Whitelists may or may not factor into her decision to see you as a client. But, hey it couldn't hurt, right?
I think whitelists are really more of a courtesy thing. They may not mean much in the big scheme of things but I think they are nice. To me it's the virtual equivalent of a pretty girl whispering in your ear "Thanks - I really had a nice time with you."
And who wouldn't want that?
;-