Well, it's fixed now - though it revealed a shoddy installation job.
I ran completely new RG6 directly from the satellite to the receiver to see what happened - still nothing: no HD locals, no signal on 99b, almost no signal (I discovered) on the 5th sat, either. So, I started looking at the dish itself. The elevation was off by 4 degrees. Fixing that brought in a strong signal on the 5th sat, but still nothing on 99b. So, I tried moving the azimuth (just guessing, since I have no compass or anything), and *poof*, I got a strong signal on 99b, too. So, I put the old cables back in place, and everything works fine.
It's possible that a recent storm changed the dish, but it had loose bolts. Worse (I should take a picture of this), the work attaching the mast assembly to my deck is completely shoddy. One of the three bolts attaching it to the deck is properly flush; a second is drilled in at a jaunty 45 degree angle and not completely screwed in, and a third is not as bad, but not true either. Also, the installer stuffed bark mulch in between the assembly and my deck to stabilize it (that's going to last....).
I can understand if he had problems in the install, but I would have preferred him to tell me (I know, I should have inspected...it never would have occurred to me to look at the bolts attaching it to the deck, and I was on a work call when he had to leave....lesson learned). This same guy didn't bring a drill with him, asked to borrow mine, and then was annoyed when the battery on mine died.
Ah well. At least it's fixed for now and I can pay my electrician (who also handles satellite TV) to do a proper job.