OK I know exactly what you're talking about.

What's going on is that some days you go in with a very positive mindset. This may be because you were confident when you left your house or maybe because your first one or two aproaches were particularly receptive. This confidence really does reflect in your body language and in the conviction with which you speak (i.e. you may be saying the exact same stuff but when you're confident you say it with more power). The mindset you are in will really be communicated to the girls and will affect your game.

So, although you may not be 'nervous' you appear to just be having a bad day sometimes.

If it is one of those 'bad days' then try to recognize this lowered confidence in yourself. Step out of the field for a few moments (maybe a bathroom or something) and just get back into a positive mindset. Think of previous successes and realise you are able to get girls even if they are better looking than you. Although it is possible to fake confidence it is much easier to just be confident.

By the time you have hooked your next set you should be back in the right mind frame.