I have had the same experience with a pool like that -- the ground has to be VERY level, because even a very slight slope makes the water higher on one side of the pool than the other. The problem is that ground seldom is that level, and even if you could get it level enough, it would shift over time.
You can try filling in the area with soil to lessen the problem. But if you really want the pool to be level, you'll need to regrade the area, or even better place the pool on a paved area such as a patio.