I do have this problem. Especially in conversations. I often times will not even start a conversation of any sort with anyone unless I have a specific question, or need them to know some info that I have. I don't walk up to anyone just to say "Hi, how's things?" The thought doesn't even occur to me to do that, so when someone deviates from the topic that I had approached them to talk about I can't reciprocate. I just freeze. I know from awkward pauses in the conversation that I was supposed to say, or do something, but I have no idea at the time as to what it is. It's only after trial, and error through repetition that I figure out the correct way to reciprocate to certain things.
For example, I've recently become aware that when talking to another woman that I know fairly well, when she mentions her new glasses, haircut ect... and pauses I'm supposed to compliment her, or at least make a comment about it. It's not that i didn't notice, or make a comment in my mind about it, it's just that I don't think about telling the other person. This seems to be very important part of building friendship between women.
It's interesting that the main reason that my 6 yo son didn't get a diagnosis of autistic disorder is because the doctor felt that he could emotionally reciprocate with others. It may not be typical, or age appropriate when he does it, but he can do it. If he has something that he finds really neat, he will at times show another person what he has/or built ect... He will also look around to see if other people are laughing when he thinks something is funny, sometimes.