This sounds like the shifting chain might be at the wrong tension. Look at the rear hub and you'll see a shifting chain that leads into the hub on the drivetrain side of the bike. If the Nexus 8 is anything like the Nexus 3 (which I have on my Trek cruiser), there'll be an adjuster barrel you can turn by hand to tighten or loosen the chain.
One of the nicer features of the Nexus hub is an indicator window; you should see a yellow indicator in there, and two lines on the window to line it up. You may need to fine-tune this a touch by riding it down the block and back until it shifts perfectly.
(In the event yours has no such indicator, try loosening or tightening the cable; the correct tension on the cable will allow you to use all the hub gears.)
Edit: Was over at my friend's house today, he has a Jamis with an 8-speed Nexus hub. The indicator window is smaller than on my cruiser's 3-speed hub, but it's there. The window is right up against the spokes.