Firrst clean and lubricate slide portion that drives nail in. Dirt will cause this sometimes. If problem persists you may have a bent driver and may have to be replaced. If you can remove front guide, check for burs or gouges on metal.
These tools are quite complicated to work on. Here is a parts diagram but I would highly recommend sending or bringing it in to a DeWalt service center for the repairs. This link is to their site where you can also find a listing of service centers near you.DeWalt DC616 diagram
It sounds like either the trigger valve is stuck of malfunctioning. It is item 68 on this diagram. With the toll UNPLUGGED from air make sure the trigger pin is releasing as it should when the trigger is depressed. Obviously do not check this with it plugged into an air line. DeWalt D51823 diagram
On pneumatic nailers, this is usually a sign of worn or broken o-rings inside the gun. If the driver isn't retracting it's usually caused by the o-rings on the lower half of the cylinder or the piston ring.
First make sure you are using the correct nails as far as gauge and angle. These are complicated tools and you may want to take it to a DeWalt service center for repairs. Here is a diagram for you if you don't have one.http://servicenet.dewalt.com/Products/Detail/DC618KA
