some have clutches and some don't, but they all have cable that runs the stringer head. either the cable has worked loose, broke, twisted, or the clutch in not strong enough to turn the head, clutch is a pretty simple devise two circular halfs cut into with offset, and spring around it inside a housing. the faster it spins the more it grabs the housing and spins the cable. at idle spring pressure colaspes it and the outer housing stops, rev it up and centrifical force overtakes spring pressure and it expands against the housing and spins the power head at the other end providing the two are connected with the wire cable????

If this is a quick link model then you have a split shaft with two cables and a clutch???

Take the engine off and check whether the clutch bell ( the drum likt the drum brake) is connect to the trimmer head by turning the bell and put resistance on the head.

If so, you connection is fine, and your problem is the clutch. look at the shoe to see whether it is all worn out, if so, maybe you can buy a new pair. For a commercial unit, there should be no problem buying this. I don't about craftsman. Check anything obvious like the spring that hold the clutch binded and stucked. Nothing is too hard to fix, just questionable whether you can get parts.