Either the fabric was cut off grain (or it was off grain and twisted off the roll) or it was sewn together crooked. I think the only way to really fix it, would be to take it apart and re-sew. You could try blocking it by clamping and spritzing and pressing, but you might end up stretching it, or if it worked, it may go back to where it was eventually. I don't think there's an easy fix for this.

You might try exchanging it, if possible.

^^^^ What she said.

The best fix is to unstitch things and straighten the grain out, then restitch.

A satiny finish slip might alleviate the problem. Are you wearing a slip w/ it?

Are your shoulders and hips even? Sounds silly but it could contribute. Stand against the wall and have someone mark 4 points. Sometimes people check to see if leg length is the same. But I agree, it's most likely cut off grain. Just return it. If you try to fix it, there might not be enough in the seam to straighten it out. Then you can't return it because you tried to fix it. Besides, I am sure you paid good money for this dress and deserve a ready to wear dress not a project.

Two ways for you!

1.Let tailor help you to revise it into the way you want.

2.Buy a new one .Referring to the experience of the first sheath dress, you'll know which style dresses are suitable for you!