I've had a few encounters lately and I felt really relaxed with these girls.

I met this girl on a train the other day. She was working with tickets and I really liked her a lot. She was definitely "my type" so I opened almost instantly when I first saw her with a smile and the fact that she reminded me of someone I knew a long time ago (which was true as well).

We met in the same circumstances a couple of more times and had some chit chat. My problem is though taking it to the next step after this. I was completely clueless as to how I would take it further and it started to feel a little awkward seeing her there.

I asked her for her number and she declined this time which made me realize that I must have missed something and it did feel that I have missed something but I just haven't been able to figure out what it was.

This is usually where I get stuck most of the time though. I can open them and talk to them without feeling nervous, but when it comes to developing and deepening the conversation, I get completely lost.

What is it that I'm missing here?

you've just gotta study some game, that's all mate

That's not really helpful Skygroove.

I've found that there is a curve- a natural time limit on "opening" conversation.
If the next step is not taken within x amount of time, the conversation will have peaked and then slide down into awkwardness. How are these next steps taken when the "vibe" is not in the on position?

Edit- Lurk moar- learn to escalate