labrador not enjoing walking

Question
i have a labrador puppy now 20 weeks old.He sometimes flops down when on a walk and refuses to get up. I dont know if he is just tired or if there is an underlying problem.He does walk with a slightly swinging gait.IS this usual lab behavior?He is fed on hills dry large puppy feed and is not overweight.  

Answer
Usually by that age, they can out walk you.  Could the heat be a problem?  any place you can let him take a dip early in the walk?  You can also wet him down.  I am frequently out in the sun all day with my dogs, and it is important o keep them cool.

He could have some problems.  I would talk to the vet about it.  Keeping him lean and feeding a good chow like the Science reduces joint problems, but genetics are still the biggest factor.  

You should switch to an adult chow now.  As you run low in puppy chow, start mixing adult until at the end of a week, you are feeding all adult chow.  It slows growth allowing sturdy joints to develop.