Welcome to the world of screwed up U.S. ATSC DTV. It is one of many things wrong with ATSC.
Try this and see what happens: dial the digital channel directly, one that isn't in the tuner's scanned list. Does it allow you to now add it to the known list?
If not, you are going to have to make a list of known channels and their OTA DTV channels.
This is going to be a real mess after the analog shutdown, as the stations do not have to change any of their on-air-look from their current channel number. For example, the local Fox47 can stay Fox47 and indicate their channel at 47-1, when in reality their OTA channel is 11. Anyone manually punching 47 will get nothing and think their tuner is broken. The government doesn't give a rat's ass that this is going to cause problems, just so the stations do not have to redo all of their channel IDs. Yes, it will cost a shitload of $$$ to change everything, but so many people that do not have a clue how DTV actually works are going to be pissed.
UPDATE: Some of the new CECBs allow multiple scans, i.e., point the antenna in direction A, scan, then repoint the antenna in direction B and scan again. The newly found channels will be added to the list. Check the manual as to what the scanning capability is of the receiver. Unfortunately CECBs only output 480i, as the purpose is to allow DTV reception for analog only TV sets/monitors. HDTV sets, or STBs, might have this capability. Be sure and check the specs, or the manual, before purchasing said device if you need to add channels that won't be found via an initial scan.