Thank you for the photos, they explain what is going on.
In the photo that shows no bottom depth there is one major thing that I noticed. The fishfinder range scale in that photo says it is set to 60ft and there is no bottom echo showing which means that the water you are operating in is deeper than 60 ft. When the fish finder is set to manual range it tells the sounder to keep the range at that depth. If the water all of a sudden becomes deeper than the range the fishfinder is set for it does not know to go deeper unless it is manually adjusted.
When running the fish finder in manual range, if you do not see a bottom echo, range the fishfinder to a deeper depth until you see the bottom echo.
Or just run the fish finder in auto range.
Kind Regards,