double check you have very little resistance at the bottom end ie cable getting dragged on the bend
i run a drab of oil down the cable to help it not snag,i dont run a great deal of tention but it does need a bit
when you do both stand up and look down and pull the lever,make sure they pull even and just with the spanner move the one thats not working evenly
double check you have springs in the correct side happens alot even to me once when not paying attention.