Note: these diagnostic procedures will apply to pretty much any Rockshox dual air fork including Reba, Reba 29er and Revelation forks.
There are two potential causes:
If the answer to any of these questions is "yes" then the o-ring on the air piston is leaking. If the answer to all three questions is "no" then damping fluid in the damping leg has leaked into the fork lowers and the rebound rod seal (possibly just an o-ring) needs replacing.
If you suspect a leaking positive air spring schrader valve or top cap then paint a water and detergent mix around the top cap and schrader valve and see if there are any air bubbles. To fix see if the top cap is tight, replace the valve or the top cap o-ring.
There are three potential causes:
If the answer is YES, then you're running too much pressure in the positive chamber. Set your positive pressure to achieve the desired amount of sag .
If the answer is NO, then damping fluid has leaked into the lowers past the rebound rod seal. You can't compress damping fluid so this is stopping the forks achieving full travel.