How can I fix my RYOBI RBC-254FC Trimmer/Brushcutter?

I have a RYOBI RBC-254FC trimmer/brushcutter which has until recently been operating very well. Now it just does not seem to have the power although the engine seems to be performing correctly. Whenever the strimmer is put to work the head just seems to slow right down. Could you let me know what the probable cause of this might be and what I can do to rectify the fault? I have been trying to get a spare parts list on your website but this model is no longer listed.
Regards,
N. Larnach