I just purchased a brand new Cannondale Synapse Tiagra 6C. I took it on a ten mile ride today, and ran into a problem. If I trim so that the bike shifts down, it is then reluctant to shift up - often it won't unless I coax it with the shifter to complete the shift. If I trim so that bike will shift up, then it won't always shift down. I can't find a happy medium. I've run into this before, but can't remember how to adjust for it. I have forgotten how to resolve this issue. Advice please???