Dense history books can kill a book club fast when nobody wants to be the one to say the chapter is dragging. A timeline printout is such a simple fix but it makes total sense that it worked, since it takes the arguing out of it and just lays out what happened. Facts shouldn't be up for debate in a group read, save the opinions for the parts that are actually open to interpretation. Six weeks for 40 pages is rough, but at least you all figured out the problem before people just stopped showing up.