D600: A gazillion EV of dynamic range isn't worth squat if your camera won't focus or meter properly Locked

Too close range for pop up flash and room too dim for iso 800 which OP used. D600 is a low light camera and you can afford to go to iso 1600 or more as it will perform well but with less flash.

Or add a SB400 or SB700 flash and bounce flash to ceiling if possible or add flash diffuser (or under power the flash, on back of flash if in close range). Just less light on subject is required. I would recommend reading up on these things. I prefer single spot AF for portraits as well, and make sure it is locked on before firing. I use Gary Fong diffuser on my SB flashes, and there is also a model called Puffer (I think) for adding to popup flash, (although it takes a few seconds to put on, if you are worrying about loosing the moment.) Adding an external flash will get rid of the shadow below subject, as flash will be higher, which is caused I believe by the long lens creating shadow from popup.