Why have my teeth stopped moving even with braces on?

I am 26 years old and have braces on for the second time as I am getting ready for orthognathic surgery. I have an overbite and gummy smile and will be having double jaw surgery. I had braces on when I was 14-16 years old. They removed two top teeth at this time to try to make things "fit" better and look better, along with pulling my bottom teeth forward as much as possible. 10 years down the road, I am now currently wearing braces again to prep for the surgery. We are having to put my teeth back in their original place since my first round of braces had been to camouflage the misaligned jaws. This time, in order to make everything "fit" after surgery, they had to remove two bottom teeth, thus leaving two gaps that need to be closed. I have had braces on for a year now, basically just trying to get the bottom teeth ready/close the gaps... One gap closed very quickly, but the other side has not been so easy. My orthodontist has been trying different ways to close the gap but for some reason my teeth just aren't moving, which is causing surgery to be pushed back more and more. He said we will keep trying to close the gap but is unsure why it is "being stubborn". Why would my teeth stop moving? Is there anything I can do (besides good dental hygiene, wearing rubber bands as directed, etc)? Is there anything my orthodontist should try?