I just had this problem and figured out that it seemed to be associated with winding on. Some of the things I found:
If the rod at the back you are tying onto is not lashed to the cloth beam it will bow in the middle and create more length once it is warped.
The second thing I discovered is that if I am not good about keeping the warp even as it is beamed it can fall off on the sides on the beam so it is fatter in the middle of the beam and sloped on the sides do to the volume of threads pushing them off to the side. This also shortens the warp threads in the middle. I usually place more beaming sticks in to help stabilize this, but they need to be long enough to cover your width. If they fall just a little short you will have your warp threads fall off the ends. Jane uses paper, so she will need to speak about how well the paper works.
I am still learning about his as my warps get longer and longer with the potential difficulty of having the warp staying the same length. I would love to learn if any of this helps and any additional ideas from the team.