Tiagra refuses to shift into higher gears
I'm just starting out doing my own repairs on my bike so I apologize if I don't know the correct terms and such...
I've recently developed a problem that my rear derailleur refuses to shift into high gears. The bike is a 2006 Specialized Allez, 8 speed cassette, 3 ring front. The brifters are Shimano Sora, the derailleur itself is Shimano Tiagra. The cable tension is tight. For the lower gears, when I shift in front the derailleur will click over to the appropriate position keeping tension tight, when I attempt to go into higher (5-8) gears, the cable immediately goes slack (and gets slacker as I shift higher). The derailleur doesn't move out as it should.
Any clues on what I should do to narrow down the problem and fix it?