why wont my strimmer run fast enough to cut the grass?

I have a ryobi petrol strimmer, with a kawasiki engine.
lately i have been noticing that it has been gettin a bit sluggish and although it seems to be spinning fast enough, when i come to cut the grass etc with it it slows down even though i have it at almost full revs. its as if it has no torque or not enough anyway, its possible to stop it spinning when i have it at full revs, is this normal? the engine seem to be running a bit noiser than usual when its not spinning as well as it should.