Mate it's wear on the little plastic cam thing that the indicator switch rests against. Remove the switch and pop the steering wheel off and and you'll see it. The plastic cam gets wear marks in it and it's just enough for the switch not to engage/disengage. I fabricated a fix by wrapping a light gauge strip of galvanised metal around the cam and lightly hammering the ends over, thereby clamping it into position. This gives the switch a nice firm edge to operate against. Problem solved for no money!