While on a channel you know should work but the screen is blank, or the image is frozen, press the “info” button on the TiVo remote. Next push the arrow down button on the remote once so you can see what channel each tuner is set on. If all six tuners are on the same channel that could be a problem for SDV channels/Tuning Adapters.
Arrow right and select a tuner. Next manually type in a different channel then press “enter” on the TiVo remote. Repeat until all six tuners are on different channels.
For whatever reason all six tuners sometimes go to the same channel, especially after a reboot.
Hopefully this may help some. It worked for me.