Getting them accustomed to using their nose IS a key, and is definitely something you should work on. To help with marking visually, you should also mix up how the marks are thrown. Get someone to throw for you and put them at varying distances, angles, and locations. Also, a handheld (little to medium $) or remote launcher (big $) will help out tremendously if you can’t get someone to throw for you. It IS what you suspect, repetition and NOT letting the dog figure out the usual distance you can throw a bumper.
The dog should get a visual mark that is pretty darn close. Once in the area, they should use their nose & hunt it out. Talk to the dog when you know she doesn't have a perfect mark. FIND IT, FIND IT. This is a cue to tell them to go into hunt mode.
Just my opinion...
"I'll start spending less time with my dog and more with my wife when she starts fetching ducks for me"