Trimmer head won't rotate

Probably something wrong with the mount. You have a photo?

A couple of things here, are you sure the two halves are securely together? if yes you need to check the squares of the drive shafts at both ends, these are flexible drive shafts squared into the gearbox at the bottom, and into the drive drum at the top, if they have rounded off they will slip and stop altogether if the trimmer head is loaded up.

if the start cord is on the shaft side of the motor , there is no centrifugal clutch to break the rotation of the head so the idle speed is not the problem
it is a design problem and you can move on to a better unit that does have a clutch

Be sure something is not obstructing the rotation of the trimmer head. If something like a small vine or twig is jammed inside the head it would make a noise and have very little rotation. If the motor brushes were worn you might get the same symptoms.

Remove the shaft and pull the drive shaft cable out and check to see if its broken or stripped at the ends.

on top of the head there is a star shaped bolt(middle of the head), remove bolt and rotate the spool you should see the gear turn, to remove the head from the shaft remove bolt/screw on the side of the shaft towards the top of the head, remove head and inspect the shaft, this should rotate when you pull to start the weed eater, if it does not then you will need to remove the top portion of the weed eater and inspect to see if the shaft is broken or not seeded inside the motor. Hope this helped Tim

you will need to pull the head off, as it sounds like the cable is off. there should be four bolts/screws, there maybe some in the middle also, gently remove the head(this is the top of the trimmer where the pull cord is, once you have it separated you can turn the cable by hand and the line feeder should rotate, if it does then look inside the head and make sure it is still square and not rounded. or you can pull the line feeder off and pull the cord and see if the cable rotates, whichever you feel comfortable with trying. Good Luck Tim

Have an assistant start and hold the trimmer. At idle, there should still be a little drag on the clutches to make the shaft spin on that split shaft trimmer with the accessory head removed. No need to rev it or do anything fancy, but teamwork will make it safer.

Now forget about the engine and clutches - does the accessory head spin freely when turned by hand (removed from engine still!) and with a clear rotation of the shaft end visible inside the connector socket? Is the shaft end square with sharply defines angles, or rounded off? Does swapping in a different accessory function normally and as expected?

Sometimes the nub end of the shaft wears. Worse, the socket on the engine side has eaten itself to pieces. Worse, worse yet, clutches in the engine side that engage the accessory shaft are worn and there is simply no transmission of power. Good luck, hopefully this info will set your boots in the right direction!

If the head does not turn, but the motor runs freely, then the reduction gear is damaged. It is inside the motor housing. If the motor tries to run but does not rotate, then there is likely material bound up between the moving and non-moving parts.

MANUAL:
http://www.dewaltservicenet.com/servicenet_home.aspx?AspxAutoDetectCookieSupport=1

PARTS:
http://www.ereplacementparts.com/black-and-decker-grass-trimmer-parts-c-4167_9518.html

the drive cable probley kinked or bent and does not drive the head the cable goes from the motor to the head take the clamp bolt out and lift the head off the cutter check the cable