The State of Mind has already discussed this effect.
A girl is attracted to a man, but she already has a boyfriend herself (and so may he). When she meets the man she is friendly, and full of flashing eyes, wit and repartee - the interaction between them is high. But at that moment she is committed - so the relationship goes no further.
On another occasion when she no longer has a boyfriend (it may be before or after the above occasion) she meets the man she is attracted to. Now she could go out with him, and she is tense. Her eyes don't flash, she hasn't much to say for herself, the interaction is low. The man interprets this as her not being interested, and nothing happens.
She is approachable when not available - but not approachable when she is available.
Most girls will show elements of this syndrome, as discussed in the State of Mind, but not necessarily as extreme as described above. But this extreme example is not necessarily due to some inadequacy or shyness on the part of the girl (though this may be a factor). It may be because she is very attracted to the man. For this syndrome can be observed in girls who have well above average ability to get boyfriends. It is the reaction to her very high interest and attraction.
Very high attractions generate high tensions which have been discussed under the Catastrophe model. When no dating is expected, the tensions are relaxed, and the interactions high. When a dating situation occurs, tensions rise and interactions reduce. (Variants of this syndrome occur when dating is precluded by situations other than described above.) We shall have more to say on this topic under Beautiful Girl Syndrome.
Thus if this syndrome is met in the field it should be taken that the girl is very interested. In the first place if a girl exhibits the above behaviour - of flashing eyes, friendliness, wit and repartee - it is indicative that she is very attracted to the man. So if one meets her subsequently and she no longer shows this behaviour, don't take it that she's no longer interested - it almost certainly means she's now available.
A simple offer of a date is all that is necessary. She may not react with enthusiasm, but she will accept. Don't worry here that a date is being offered where high interaction is not taking place. Very high interaction has taken place previously, she has got to know the man, and that's what is important.