How to make a better marking dog, what I or anyone in retriever competitions would pay to know the answer

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Without seeing what's happening in person its pretty tough to tell if its a problem or just enthusiasm.
Make your marks hard to get to but easy to find once they get there. IE Throw the bird so the dog has to cross some heavy cover but once he gets to the area of the fall have the bumper really stick out. A 3" white bumper in 2" grass is hard to miss. In addition try to throw 75% of your marks with the wind so the dog doesn't get rewarded by running at the gunner.
A good drill to work on marking is the Y drill. I could try and explain it but youtube has lots of videos that will do a better job than me. I think Bob Owen's has one of the simpler videos. While I don't think Bob is much of a trainer his Y drill video is pretty good.
Also try to find Hillmans stand alone marks drill, these can be combined so you can throw marks without a helper. My guys get this at least 1 time a week.