Ah the age old question, the game of cat & mouse. Every client wants it, but they can't cum for everyone.
Rare occasions will be if you two click, if there's a spark, if she's enjoying ur company, it's happened to me twice, on both occasions they were enjoying my ass that much & they could see how much I was too that they just kept going to climax, but it wasn't for my benefit, I didn't see it. Still I took it as a compliment
The only way to make sure is to pay for it, it's not going to happen in an half hour session (well not visually anyway), in general at least an hour session is the norm, some will charge more.
They only one I know that seems to be willing and able to release load after load is Mrs Rammer, but I don't think she goes up north all that often :/, but you could always ask