Thank you for posing this question to me!
It's great to hear the male side of this situation, because I've definitely been the female in a scenario like this.
Usually the door will seem closed if she leaves and no numbers have been exchanged and no plans have been made. But that's totes not true. A woman who has visited your shop more than once, and has referred to you for help on all accounts, is interested and will remain interested. However, I do advise at getting the ball rolling the next time she visits, if you are indeed interested in her. The reason she will not be the one to ask for your number, but instead will go on and on about how helpful you are, is that she feels that after giving you the compliment, it is a) not her place to then also ask for your number and b) she assumes you are "on duty" and maybe this is your work persona, and if you were interested in her, you'd have asked for her number already. When you "let it go" as you feel you do, they probably are not disgruntled, upset, or think of you in a bad light WHATSOEVER! A girl will never think that of a guy who doesn't ask for her number just because she showed interest, especially in a work place. She'll brush it off as you being more professional. Even though she totally wants to make plans with you
If anything, she'd admire for you to sticking to helping her and always welcoming her. But yes, they're waiting for you to make the next move, because, when you think of it, do you ask for your cashier's number? On either side of the spectrum it'll seem awkward, as there's a supposed business attitude and then personal attitude, but if you prove to these women you're genuine when you work, then you'll have no trouble at all shifting from the customer-employee relationship to dating-or-friends relationship.
Good luck =) And let me know if you'd like more clarification =)