Humminbird Questions

[NAV165]

I'm finally getting around to trying out everything on my boat. I have a Mega 10 on the helm and a 9 on the bow. They are linked. When I'm using the bow mount unit I can see my waypoints but not the depth contours, is this correct? Or do I need to have a mapping chip in both units? Also for some reason the bow mount unit depth is frequently off by 10 feet or more compared to the depth reading on the unit on the helm. I've raised and lowered the trolling motor to see if the transducer is coming out of the water but no change. I know it is not reading the depth correctly because the unit on the helm will show 8 feet and I can see the bottom and the unit on the front says 20 feet and I can still see the bottom and I know I'm not on that steep of a drop off nor can I see the bottom in 20 feet of water. Any ideas?