Adjust your limit screws.
If that doesnt help (I bet it will) you may have a physical limitation issue.
I use a short cage 9 speed chorus rear deraileur. its maximum cog capacity is 27. at 28 the cage and chain rub against the largest cog
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2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
2000 litespeed Unicoi Ti, XTR,XT, Campy crank, time atac, carbon forks