Hey man!
First of all, great post! I like people who put real life effort into this.
So, i read your convo's.
They are all good openers, but i think you just lack real interest, because you are nervous, because it didnt really went the way you wanted it to go, perhaps because you was too occupied with being C&F.
It is obvious that you lack sincere interest at conversation 2 and 3 (the hot blonde in the jeans and the hispanic lady). Hell, even the 15 yo was good for conversation. If it is learning conversation, a 15 yo is a nice surprise, kinda takes you off guard

, aint it?
but convo 2 and 3 are the best examples.
So what is happening here at convo 2:
First, you put on a big smile even before she actually deserved a smile. What i mean is, she didnt make you smile because she was genuinely funny. So approach with just a friendly open face and give her a chance to make you smile.
secondly: she replied: this city... and you didnt hook up on that. Its not that you was satisfied with that answer, but you didnt push just a little further. you could have done worse by asking if she was from around, etc etc. shooting questions at her. Which would indicate her again that you arent really interested in her answers, which means you ask random questions just for the sake of asking questions.
When she replies: this city, look at her like you expect much more. Unless ofcourse you arent really interested in the jeans (be honest here), you could have said after a few seconds with a look on your face like you expect a good answer:
like... this city... someplace particular or did somebody hook you up? I mean, they are tight jeans, they must be black market stuff! How much did you pay for a kilo of that tight jeans?
The point is: when you ask a question, expect a real answer. not something like: 'this city' or 'Spain'. Expect more and tell them you expect more! Spain? wow, the thing i like about Spain is Picasso!
she: really? why? because of his art?
you: His art? hell no, he was a real womanizer! you didnt know? (look suspicious) are you really from Spain?
and there you go. From here on, you can start asking more personal questions, slowly during the convo, amp it up. Have an interest in what you are asking. Take their answers. If they are bullshit answers, tell them you need more to work with. What do you really want to know? take their answers and continue from there. If another topic jumps into your mind, FORGET ABOUT TRANSITIONS. You dont need transitions, just ask the question, or tell your story! Give your opinion. Be honest. If you are genuinely disagreeing, people will like you for it.
Example with the jeans:
you: hey nice jeans!
her: thanks!
you: where did you get them?
her: this city
you:like... you can get them at every corner or was it more something specific...

?
her: (smile) abercrombies.
you: holy shit, i dont like that store!
her: why not?
you: because its flooded with half naked men trying to sell you stuff i dont need! Its like a Twilight fanday!
her: haha, well i can imagine you dont like it... but i do!
you: omg, so you are a dirty little girl arent you!
etc etc etc...
You see? you dont have to agree with everything. To make a connection, you just need to be interested and congruent to your interest. Congruent means: dont ask about jeans if you truly dont give a shit. If you start acting like you suddenly do like it, not congruent either.
Give those girls a chance to be unique as well. Let them show you they are somebody, so you can compliment them on it if you really like it.
example, the two set with the spanish girl:
your: hey! you arent from around are you?
hb 6: i am, but she is not
you: oh really? where are you from?
hb 8: Spain
you: ... ok, but Spain is a big counry... could you be more specific?
hb8: well, Madrid, i live in Madrid, not far from the Prado museum.
You: oh, I know the Prado Museum. There is a Picasso hanging there!
hb8: yes i know. I like art and i like Picasso!
You: oh now that is cool! I like people who are culturally involved! But i dont like Picasso though...
I hope i am making sense here

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Also, take it easy on the attraction. Focus on the interaction instead. Make connection. When you had a fun conversation, made a connection, you can say something like:
hey, i had great fun actually. You are cool/interesting/nice. I want to see you again, lets go for a drink tonight, saturday, sunday, next week, whatever.
Also, one more thing that comes in my mind: just stopping people in the middle of the street is strange. Especially if you start talking about jeans or something. On the streets or in public, be a little more straightforward.
Example with jeansgirl:
you: hey, nice jeans!
her: thanks!
you: where did you get them?
her: this city.
you:.... ok... heheh not very specific, but that is ok. To be honest, i only asked to have an excuse to talk to you. I think you look nice and i want to get to know you more. Lets grab an icecream tomorrow?
Like that. I think i missed out on some more topics but i hope i gave you some ideas to work on.
To summerize: GENUINE INTEREST

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Cheers!