From McCall’s Sewing in Colour
When sewing a curved seam, such as an armhole, or neckline, it is important to grade and clip the seam. Curved seams ‘bunch’ after they have been sewn, & even ironing doesn’t help to make them lie properly. This is because the seam needs to be clipped to allow the fabric to expand or contract as it needs to so it can lie flat.
Firstly, trim one seam (normally the bottom one) down to about .5cm or closer if possible. Secondly, trim the other seam down by about half.
Lastly, clip in to the seam as illustrated, every few centimeters as close to the stitching as possible without snipping the stitches.
Iron the seam flat using a Tailors Ham, or carefully on the ironing board & there should be a noticeable difference.