I replaced the cassette on my powertap wheel (from 12-28 to 12-27) however now the chain won't stay in the big cog in the back. When i press the shift lever, the derailleur moves the chain to the largest cog but then the chain moves back to the second largest cog when i release the shift lever. I don't think it's a cassette issue as I occasional switch between training and race wheels. not sure why this is happening now. I tried adjusting/increasing tension on the cable using the barrel adjuster however it still doesn't work. I also tried increasing cable tension by undoing the cable bolt on the derailleur and pulling the cable tighter however that doesn't seem to fix it either. any thoughts what could be the issue?