You're not alone, trust me! It's difficult to work out why girls; oh sorry, ladies do this. Some do it because they like the attention, its a big ego boost for them, when they know you're interested; its job done, fall back, lets see if I can do it to someone else.
Some ladies are just naturally flirty, they don't recognise they do it until the guy shows interest back, then it's fall back, lie low and hope it blows over.
Some ladies enjoy the whole flirting game, it's a fun past time but forget the consequences of their actions, although a lady might like to flirt, she might not be ready for a relationship...at first flirting may seem harmless but in the future when there's a chance to get more then seductive eye contact they realise "I'm not ready for the next step."
Saying this, some girls don't like the flirting game either, they may even like a guy but afraid that a relationship (if went wrong) would spoil what they already have with the guy, not worth taking the risk.
Risk is an important factor here, because it may have something to do with you're approach rather then the lady. Are you getting too close too soon? Have you build any trust with the ladies in question? Do you say or show your love to a lady? Whatever the case, you need to do it with minimal risk to them. Ladies have a self defence mechanism which puts McAfee to shame, it's also better to show a lady you love them before you actually tell them, this way not only does it make it easier for you to tell her; by saying what great times you've both had together, it's a why for you to find out how the lady reacts to your show of affection.
Problems with your approach? Have you told them everything about you in the first few weeks, in which case the lady doesn't need to find out anything herself? No mystery for her, no reason for her to pursue. Moving too fast can be considered being 'desperate', I said this in another post but showing too much love before a relationship is considered creepy, but during a relationship is considered sweet. Think about it, if you hardly knew a lady who you knew was trying her all to get you to like her, first you'd want breathing space to ask yourself what you should do, then you'd want time to get to know her better before you make any decisions.
All the best in the future.