why dosent my directv receiver show some channels at times?

[Nick]

okay so ive been trying to watch certain channels but all i get it a black screen (comedy central, FX, amc, nicktoons, tbs, ect.). this is not always the case but it dose happen very often. when i press the guide button the little screen on the top says searching for signal (711). i have pressed the reset button and pulling the plug out for 30 seconds but still no change.

thanks in advance for your help
-Nick