I had to replace the barrel adjuster and shifter cable for my rear derailure. I screwed the bew adjuster all the way in, put my bike in "7" (all the way) with the chain on the smallest cog. I pulled the cable tight and secured it with the bolt. Now, it will shift up and down in the smallest gears fine but if I get her onto the big rings, the cable just stays slack and the bike doesn't downshift. WHat is causing the cable to go all slack like that?