Something else that happened when I got caught up in getting the plot finished was that I neglected the world I was building. I left a lot of questions unanswered that, according to OSC, have the ability to shape and enhance the plot. In his book Writing Science Fiction and Fantasy, he recommends asking "what if" questions, which will lead to other questions, until you have established firm rules for your world.
That was one of the things my plot critiquers mentioned- they got confused about the rules of my world. When I looked at the questions they were asking, I realized that they were confused because I didn't know the answers. So I started asking what if, and focused on writing an explanation of the rules of my world instead of a plot. After a week, I had four and a half pages of explanation, and there were still some questions left to answer!
The fun part was that some of the answers to "what if" that I came up with really did open up the plot. Some of my characters changed dramatically, which led to new possibilities for the plot. Certain aspects of the world I built made plot twists possible that hadn't been there before. Now I'm really looking forward to working on the plot- I'm no longer constrained by what I don't know.