Tvheadend

It could be that you are using a bouquet or other automatic channel mapping. If you attempt to play the service from the WebUI, does it play? If so, then you will most likely need to manually assign that service to a channel. If it doesn't, it could be that you do not have access to that service, either because of encryption or that it does not contain a TV program.

I am not using either of those features.

No, I know these have channels, because I can see them in the service list. For instance, on 527 MHz I have five channels, all of which have service names, quo, ION, etc. However, when I select any one of these and try to map it, I get nothing.

In my Services list, they all have Play links, as do Muxes and Channels. Check the column display to see if the column is there but is being hid.

And just because a Service was found on a Mux, it does not guarantee that the Service carries data/TV Programs, either presently or in the past.

Ahh of course, it was hidden.

These channels I am sure carry stations, because they come in when I plug the antenna cable into the TV rather than the THV box.

Then I suppose your only option is to manually map them. I believe if they have names, you can name the Channel the same as the Service, then it should automap them, but I'm uncertain as to that.