Hi Gayle,
You ask a very good question about class reactions. I don't have a good answer to your question though I have experienced it many times. I often teach the two sections of the same course and find one will be up beat excited and very interactive. The next class will be as you mentioned yours was, dead and non-responsive.
These kinds of reactions really puzzle me because I like you have a bag of tricks I use that are always successful in getting students ramped up and ready to go. The non-responsive low energy classes just set there and stare. What I do is to break them up into groups and give them assignments so they will start talking among themselves. This generally gets them going but they never really get to where the high energy class is.
Gary