Ups and downs

I'm a little disappointed.  I've been grading short essays (2-3 pages) and quizzes for the last two weeks.  It was my students' first essay and second quiz. 

Class #1 - quiz average went from 77% on the first one up to 81% on the second one, and their paper average was 83%.  

Class #2 - quiz average went from 76% to 87%, and the paper average was 81%.  

Class #3 - quiz average went DOWN from 78% to 69%, and the paper average was only 76%.  

What's frustrating is that that third class is my favorite.  I love talking with them, they try really hard, and they are just fun to teach, in general.  And yet their grades are going down.... And I'm not terribly sure what to do for them. *sigh*