Are you sure that you are increasing at the proper rate? After that first row where there are virtually as many increases as there were original stitches, the increase rate goes down quite a bit by only increasing at each edge and on either side of the center, and only on right side rows. That's the only thing I could think of that could possibly be going wrong for you, like if you're maybe increasing on every row rather than every other row??
Would it be possible to post a picture so we can see??