Or give us more details here. Do you more than one HD Receiver? Are all of them doing this? Exactly which HD channels?
99% of these problems are caused by dish misalignment. Please post back with your signal readings for each satellite's transponders (and both tuners if applicable). You'll find these readings under "Menu" -> "Setup" -> "System Setup" -> Satellite -> "View Signal Strength".
Fill in your readings for each satellite and tuner using these examples:
Satellite transponders (32 total at 101º)
[Most of your standard definition channels are beamed from 101ºW]
1-8 # # # # # # # #
9-16 # # # # # # # #
17-24 # # # # # # # #
25-32 # # # # # # # #
Satellite transponders (3 total at 110º)
[Several HD channels are beamed from 110ºW]
1-8 NA NA NA NA NA NA NA #
9-16 NA # NA # NA NA NA NA
Satellite transponders (11 total at 119º)
[A few HD channels, local SD channels, and Spanish package]
17-24 NA NA NA NA NA # # #
25-32 # # # # # # # #
Satellite transponders (6 total at 99º(s))
[Local HD channels for some cities]
1-8 # # # # # # NA NA
[Note: these can be very slow to appear]
Satellite transponders (16 total at 103º(s))
[Local HD channels for some cities]
1-8 # # # # # # NA NA
9-16 NA NA NA NA NA NA # #
17-24 # # # # # # # #
[Note, these can be very slow to appear]
Satellite transponders (16 total at 103º(c))
[Most national HD channels]
1-8 # # # # # # # #
9-16 # # # # # # NA NA
17-24 # NA NA NA NA # NA NA