OK, I did some investigating and both (fortunately, the only two) channels I am missing are on 110, transponders 10 and 12, respectively. Here's what's weird. When I go to channel 494 on my HTL-HD, I receive the appropriate message showing that I'm receiving the satellite. When I try the same thing on either of my two SD TiVos, I either get the "Channel Not Available" message or the message about "Searching for Satellite In 2" (or whatever). I then go into the SD TiVo signal test and I get approximately a 40-something reading on transponder 8, and zeroes on transponders 10 and 12.
So, my follow-up question is this: Does the fact that I can get some, but not all, of the transponders on 110 indicate a bad LNB? Or a bad multiswitch? Misaligned dish? Or something else? As I noted in my original question, my reception varies depending on the time of day. In the morning, I can receive the two channels in question (HDNM, UHD) steadily. Afternoon and evening, no signal at all. Is heat somehow a factor?
Any help is greatly appreciated! Thanks.