Hi Sharon - I have this issue before, and I figured out that I wasn't being fast enough when I was putting the final knots on the warp bouts on the front beam. I would start right to left. By the time I was tying the final knots on the left side, the right side would sag. So these days I still start right to left, but I skip the first two bouts and then come back to them when I have finished. That way they have a very similar to the left most ones. I also learned to put a much stronger knot, which helps prevents slack.
The other thing that could be happening is that your packing paper on the back beam might be spiralling from the right side to the left side, causing unevenness.