Originally Posted by HillRider

Locate the problem. Disconnect the shift cable at the rear derailleur and then turn the crank by hand and pushe the rear derailleur in by hand. If it goes all the way to the largest cog (or perhaps beyond because you backed off the L limit screw) then the problem isn't with the derailleur. If you can't push it all the way to or beyond the largest cog, the rd is the problem.