In the video setup menu, change the IR speed from 'Fast' to 'Med' and see if that works.  This is how I need to have my 8300HD configured for reliable IR commands (works perfectly at this setting).

I have mine in the middle of both of my boxes and both set on fast with no problems..  Do you have it right up close or back a little??  Mine are a good couple inches in front of the box.  Does the power test work during setup?  You might use the camera trick to verify it's outputting..

http://www.slingcommunity.com/article/10342/Use-your-camera-phone-to-connect-your-IR-emitter-/

Slinging Tivo Series 2, Tivo HD, and TW cable from a Pro. Slinging Tivo HD from a Solo, Slinging Humax DRT800, and TW cable from a Classic to Sprint Mogul, AT&T SX66, N62, 8525, and Dell Inspiron 8600 with Sprint EVDO over Windstream DSL.