Nope. Definitely NOT normal. I'd like to say that I'm surprised that the retailer told you that "all king size beds have a hump" and that its "caused by the 2 piece foundation" but sadly this is all too common. Sounds like you got a defective mattress and you have a legitimate warranty claim. The mattress needs to be replaced. If the retailer won't do anything for you, contact Beautyrest directly at 877-399-9397.

Pictures would be great. There is one thing you also want to check which is the support underneath the bed. If you have center legs that raise the middle higher, that could be a cause of this issue. It's a good thing to check this so you can be certain it's the mattress at fault when you go back to the retailer you bought it from.

The mattress you bought has a layer of micro coils in the upholstery. It's very difficult to ship these beds correctly and if mis-handled in anyway on the trip from Simmons to your bedroom, the upholstery can shift around. If the hump is prominent, this would be a manufacturing issue and something you need to go to the retailer to resolve ASAP.

I agree with Sean that no retailer should have ever told you that a "hump" is normal, or that sagging should be expected after only a few weeks.

It is hard to assess the severity of the situation from your description, but if it is as bad as it sounds, I would add that this bed probably should never have been sold to you in the first place. If you replace it via a warranty claim, you may not have any further warranty protection on the new mattress you receive (it depends on the terms of the warranty on this model), which doesn't seem fair if a mattress was delivered to you that was already known to be defective.

Can you post a picture here that shows the condition of the bed?