No installer can guarantee the seam will be invisible. Think of carpet as a textile. you see the seams in your clothes-- and you may see them in the carpet. The carpet & rug institute www.carpet-rug.org publishes the standard for carpet installation-- and includes a statement to this effect.
Some carpets are more prone to this than others.
Now there are installation or manufacturing related. Like a cut not made properly. A gap in the seam tape. A difference in color (side match issues). Fibers caught in the seam.
Call back the retailer and ask them to review the issue. They may be able to roll it with a star roller, or rework it a bit.
