New TV.......Not so good HD picture

I appreciate any assistance I can get with this.  I recently purchased a Samsung 46' 3D TV that is on the qualify list Directv has.  Now, I have PS3 and Xbox 360 hooked up and the picture on the 360 is great and the picture on the PS3 Blu-Ray is phenomenal.  Furthermore, I stream Netflix through my PS3 and the picture on that is unbelievable.  But when I watch Directv, most of the channels I watch, the picture is poor.  Some channels are good like NFL network and some of the Showtime channels but most of the other channels just look bad.  I always watch HD channels.  When I push guide, I go directly to HD channels, I don't even bother with the other channels.  I have the HR21 receiver and the Slimline 5 dish.  I know it's not the TV because like I said, the picture on my other components is fantastic.  Does anybody have any suggestions?  I can list my reciever settings if that helps. 

Thanks