i don't think your training too much i think your getting giving too much direction to the dog when you do train. (like ACE said.) i also agree with phillipstd, go back a few steps and work the mark angle quite a bit. you'll wanna start this in a relatively clear area so the dog can see your bumper and get a good visual mark. make sure the dog is marking the bumper when it goes down by enlisting someone's aid in throwing them (or get a remote launcher). this will give the dog a different look than just you throwing. i use the Mark command before each bumper is thrown just to let my dog know when to pay attention in the field. after a couple of weeks of that, then move to an area where the marks are a little tougher (any cover - tall grass, brush, woods, etc.). throughout this process, do not help the dog find the bumper. just ignore him altogether until he starts the return with it. if he can't locate it, heel him and make him start over. the biggest thing is don't let him look to you for guidance for a while. imo, the only time you should handle a dog during the retrieving process is on a blind. if the dog got even a half-way decent mark, make him find it. hth!
Talk 'em into givin' up!
It's easier to get forgiveness than it is permission!