I don't know about not working correctly - it sounds as if it's not working at all.
How do you know that power is getting through fine if the motor doesn't turn?
Assuming that power is arriving at the machine as you say then there's not much to consider - any external connector - the control switch - internal wiring (very doubtful) - the motor itself.
Amazon have strimmers from £22.49
Bob Flowerdew, September 2014