Can't change from big to small ring
I have had problems changing from big to small ring. Firstly after adjusting cable it worked for a week then would not change again. My local lbs suggested a new front mech which they put on. It worked fine again for several weeks but now is stuck in big ring once more. The cable goes slack from taut when using lever but chain remains in big ring. I've had a new cable new front mech could it be the sti lever ? But I assumed if the cable goes slack it could not be that, any ideas.