Lab lost interest in retrieving at 16 weeks

Question
We have two male black labs, brothers, age 16 weeks.  The smaller of the two "Titan" has suddenly lost interest in retrieving...not good since he he slated to be the hunting dog.  I have tried attaching bird wings to one dummy which was also injected with scent, along with a plastic orange "bird" and rubber/plastic duck.  Is this a developmental issue.  I have read that at this age the environment becomes the most important thing in their life; he is easily distracted by people, planes, birds and other noises because he is very alert.  His brother "Tucker" happily retrieves and is a good ten pounds bigger.  Are they maturing at different rates?  I would so appreciate any advice or thoughts on this.  Thank you.  Kathy

Answer
You could be right about it being a stage, but it worries me.  Does he ignore it even if his brother is bounding off after it?  

Is his energy level otherwise OK?

If you hold the chicken wing in front of him and move it back and forth, do his eyes follow it?  if not, he could have a vision problem.  

Perhaps the best thing is to let the vet have a good look at him telling him everything you can.