I haven't worked on that model but the main failure point on "two piece" shaft trimmers is the coupler in the top flex shaft (that the bottom flex shaft fits into) gets stripped. Depending in the model you can just replace the upper flex shaft or you may have to replace the complete upper shaft.
Other points of failure is the clutch bell gets rounded out where the upper flex shaft fits into. It could also be the plastic insert inside the shaft tube (that the flex shaft rides in) becomes melted or shifts allowing the flex shaft to move up or down within the tube and lastly, the flex shaft itself can break.
First look carefully at the coupling in the upper flex shaft and see if there are any signs of rounding off or wear.
Disconnect the attachment, insert a screw driver tightly into the coupling in the top flex shaft and carefully start the engine. Hold on to the screw driver and slowly increase the RPM. You should feel the shaft try to rotate. If you hold onto the screwdriver firmly so it won't turn you should be able to really be able to load down the engine. If the engine freely revs it is either a bad clutch bell or the plastic inner sleeve is melted or shifted.
Now pull the shaft out (using the tightly jammed in screwdriver) and try and rev the engine. If it now easily revs it most likely is the inner plastic sleeve.
Two piece shafts can be a real pain. Some have so little engagement in the clutch bell, between the shafts and the gearbox that any shifting of any of the shafts disengages them. Your symptom of running slowly before it stopped working makes me think it is more likely a stripped coupling or stripped clutch bell. Whatever the case, it can be fairly expensive to fix.... by their pricing, Homelite must be pretty proud of their products