So I just got this bike, and it seemed to be working fine - but it only shifts between 5 of the cogs on the rear derailleur, when it should be able to shift through all 9. The component in question is a shimano XTR sgs. At first I could only shift into my top 5 gears. Then I tinkered with the two adjustment screws on the xtr - to no avail. When I tightened the cabling from the shift levers, I was able to reach all but the bottom two gears in the back, but the top two became unreachable, still stopping me from using 4 of the cogs. What is limiting the derailleur motion?