Hey man, that's a natural way that sets work. Several sources have noted that you should go into a set expecting to carry around 90% of the first 5 minutes of the conversation. Sinn did a nice document a while ago that you can download for free that talks about this and a few other really good items. Anyhow, that means you've got to be confident talking to "mutes" where the most you'll get at that early phase are short responses or comments. Now you can dramatically shorten this time, but in any case, it's not a matter of just blabbering on until you hit a certain time limit; you've got to be doing several things before you can get them engaged:
- 1) Be interesting / entertaining and have good energy. Good stories, routines, observations are important here. Think of when a stranger is talking to you: if they aren't saying anything that perks your interest, you'll tune them out quickly.
2) Demonstrate your value (DHV). You can't come off like you're bragging or preaching, but you want to be talking about things that obliquely show you are an awesome dude that they are interested in getting to know better and opening up to. In other words, build up attraction.
3) Develop connections / rapport. It's critical that you open doors that allow her to get involved in the conversation! Remember, you're just a stranger that she knows nothing about, and it's likely that you're not the first one to approach her, so even if it doesn't seem fair, it's your job to give her a reason to talk to you. It's very important to listen and be very observant; ask some good questions that give you some insight into what she's interested in, then start guiding the conversation towards things that she will want to talk about, and she will get more comfortable opening up to you.
Anyhow, it's not easy, and there will still be lots of times where you're just dealing with a dead fish who's either not interested or just plain anti-social. However, if you manage to do the above things, the conversation should start to become more reciprocal. It's usually not a dramatic switch-over: instead, as you're building up on the attraction/comfort, you should find that the girl (or others in the set) eventually start opening up.
Anyhow, hope this helps a little...