Even when I was a JW, it used to bother me that their study program was so shallow. We used to go over the same books over and over again in the book study. That made no sense to me. If we had covered certain material, why did we not go on to more detailed or deeper material? I reasoned that, in school, if I passed Algebra 1, they didn't say to me, "Well, there might have been some points in the course that you missed, so we're going to have you take the course over again to be sure you really understand it." No, they would advance me to Algebra 2, or Geometry, or whatever the next course in the progression was. By learning the more dvanced concepts, the earlier concepts from the previous course would become second nature. But the Watchtower simply recycled the same material in the meetings repeatedly.
The excuse they offered when I asked was either the one about repeating the material in case some points were missed, or else they would say something like, "there are a lot of new people in the truth now, so the material is being re-considered for their benefit." Well, that's great for them, I would think, but how about some spiritual food geared for those of us who have been around for a while?
Now, of course, I realize that they did this because there was no deeper material available. Any really in-depth study of Scripture would have shattered their belief system. The object was not to have true "Bible students," but organizational drones. I think they may by now have sufficiently dumbed down the organization to the point where anyone who really craves serious biblical material has been weeded out, and they are left with the mindless and unquestioning.