Make sure there is nothing under the ejector, keeping it from fully seating forward and flush against the cylinder.

On a used gun, this is usually un-burned powder.

However, the new 617 I just bought had a tiny metal chip under the ejector, that managed to keep itself in place until I tooth brushed it out.

I've also had copper plating come off bullets, and get stuck near the forcing cone, partially jamming the cylinder (although I believe you said yours is unfired). In any case, you can hold the revolver against a light and watch the cylinder to barrel gap as you dry fire to confirm no rub there.

You may have a more serious problem, but I thought I'd throw a couple easy ones out there.

I don't think you want to open a new gun, but I caused the condition you describe in my 629 when I mis-installed the tiny spring at the bottom of the hand, such that one leg was out of position. It appears this caused the pushrod going towards the rebound block to be slightly out of position, and thereby bind or rub as it traveled.