I think the difference in how we see Catherine's early interactions with Henry and Isabella has to do with the nature of each of her companions.

Henry may be teasing, but we can tell that he is doing so honestly. Isabella, on the other hand, seems to be hiding behind her excessive compliments, protestations, etc.; just look at how she declaims any interest in young men's company in chapter 6, yet seats herself in a prime viewing spot and then chases after the two guys who had been giving her the eye.

Catherine does not respond to this the same as she does to Henry's teasing because Isabella is not being transparent while Henry is. At least, I think that is what is going on so far.