retracting dog guard/load cover

[flying haggis]

Help! the retracting load cover and the mesh dog guard in my 211 e class estate have both stopped retracting automatically when pulled out. the load cover is usually fixed to the runners that lift it out of the way when the tailgate is opened so when it doesnt retract and remains taut it drops across the load. does anyone know if the tension mechanism can be got at and how? as it is starting to annoy me that it doesnt retract