Do you see the 'seams' or what exactly? Describe in other words what you see.
Yes, cracks or sags should be fixed by the builder. Settling cracks can be fixed by him too but I'd make a 'running list' of what's wrong w/the house and submit it just before your warranty expires. (I'm assuming you have a warranty. If not, mail him the info on any and all problems as you get them, certified mail - return receipt.) And log any phone calls or in-person conversations. Take pictures too. Close up video with date-and-time stamps are best.
The angle of the light that hits the ceiling, the color of the paint, and the 'finish' job can all affect what you see when it comes to drywall. For example, in my home in the 2nd floor hallway, you can see my 'fixes' to the nail-pop problem I use to have. But in the evening, you can't. The AM sun hits the walls at 180 degrees, thus, showing any 'shadows' from my spackling fix. They're painted white so they show even more. The ceiling is the same way.
Keep on informing him in writing and keep a log in case you end up in court. Maybe some other folks have some advice. My best to ya and hope this helps.