Well, there isn't anything obstructing it. And, when I bring either seat back down, I can hear it latch. Maybe it just isn't designed to go completely flat? The owner's manual doesn't say anything yes or no about the third seat being level.
If it's not flat it's darn near flat when ours are folded. You've got something obstructing the seats. Maybe moving both second row seats with the lever like you are getting in the 3rd row, make sure the 3rd row headrests are all the way down and have someone operate it while you watch. I was operating ours quite a bit this weekend, no special treatment needed they were very cooperative.
There's a continuous slope up from the deck starting at the rear door forward to the front seats, if that's what you are asking. If the third row seats fold to where they latch, that's as far down as they go. You can verify by folding one of the second-row seats and see how the front of the third-row deck matches up with the back of the second-row deck.