The trimmer head will not spin under power

this is a hard question ,ill start with the text book answer , NO the idle should be low enough to disengage the clutch and remove power from head . but in reality this always is not possible , me I judge a clutch and engine idle % to be working correct if I can put cutting head on ground and not have it take off with throttle closed off

Mike it sounds like the head has sheared where it goes onto the shaft. You may be able to get the head off and see if there is any way of re attaching the shaft but it may be tricky. You could use the modelling clay that is also a resin that sets rock hard. (milliput) works for me.

I'll send you a link to the operators manual. To change the cable, simply unscrew the bump feed knob anti-clockwise.

place a screw driver in the slot the drive goes into and hold the trimmer head feel the head

You may have a broken drive shaft. Some are braided and flexable and used on curved shaft units. The solid shaft is used on most straight shaft units. Remove the string head and pull the shaft out to look at the ends to see if they are broken or stripped. George

The shaft inside the tube is not properly installed either in the clutch near the engine or in the trimmer head itself. Take the head off and make sure the shaft is all the way in so that is is seated in the clutch. You may have to turn it with your fingers until you feel it go in. Next make sure the shaft and head line up. You will need to make sure they line up which you can see if you look at the shaft and head. i hope this helps./

The shaft cable has either come loose at the head, or is broken at the engine most likely. Is this a 2 piece unit? If so, see if the cable turns with the grass trimmer section removed. If so, it is usually easiest to just replace the trimmer shaft section. One piece units require disassembly at the engine, and if you are not mechanically inclined, that can be a pain.

sounds like the drive cable is jammed.you need to remove the shaft from the engine and head and see if the cable spins freely.

pull the shat off the engine and check the inner shaft it sounds like its stripped or the theengine side is cracked hope this helps

a lot of times there is a hole in the side of the head to insert a rod into so the head doesnt spin..now all you have to do is figure whitch way it turns as some a left hand thread