One may have been fished out if its a popular wreck and may need a few days to regroup. One may have more nooks and cranies for fish to hide in or maybe in deeper water, diff temp etc. The bottom currents come into play here also. I've fished on wrecks that only held fish on one side or particular section. Some species like the bottom of the wreck while others hover in the top or just above masts,rigging, cabins etc. As far as depth finders, if you can afford a colored recorder they are great, such as a si-tex or furuno.
http://si-tex.com/
http://furuno.com/