It sounds like you have lost the bump knob
http://www.ryobi.com.au/Ryobi/files/41/41e47865-aa8d-47f5-ae25-2d76ceac22d9.pdf (look at fig 13b)
For a new one contact ALM Manufacturing
http://www.almmanufacturing.com/en/grasstrimmer/grass-trimmer-parts/ryobi.html
Richard
March 2012