2 male labs hesitant to eat

Question
I have two male labs, one is 3 years old (neutered) and the other is almost 1(not neutered).  They have always, until recently, been the type of dogs to salivate heavily and bury their faces into their food when we feed them (at morning and night).  Now, they still act excited about the food, until we give them the "okay" to come to their bowls, and then they just sniff it and walk away.
When we noticed their hesitancy, we immediately switched their food from Purina Beneful to Iams Healthy Naturals, because just over a year ago a family members dog died from the contaminated Diamond Dog food.  So naturally we are a little nervous.  We have taken the older lab to the vet already... nothing was wrong.  But they are a bit lethargic lately, both are asleep right now at 10am.
They still eat their treats, so I can't help but wonder if there is something going on with the food.  I also wonder if there is some kind of dominance thing going on between them that I can't detect.  Either way, I have noticed weight loss in the younger lab, so I would like to try to find out what is going on.

Answer
There are exceptions but usually a Lab not eating is a serious problem.  Usually any healthy dog will eat all it needs unless there is a major emotional problem such as being rehomed.  While less likely with a Lab, the usual cause is over feeding.  Evaluate you dogs as in this link, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx   If they aren't overweight, and you can't think of a reason for them to be upset, better work with the vet some more.  

One other thing could be the weather.  Has it suddenly turned hot where you are?  It has finally warmed up here after a very cool spring.  I couldn't believe it a recent gas bill for $85.  Can you give them a swim?  

Don't be too quick to switch chows.  Actually it was the speciality, natural chows that had more problems with the recalls.  Iams is known for putting weight on.  It could be the problem.  You can stick to it, just cut back on what you are feeding if they are overweight by that link.  Healthy, lean dogs tend to be thinner than the average.  

There was an extensive discussion of weight in a recent newsletter from a service dog school.

''Obesity is the number one nutritional disease affecting dogs. It's estimated that 25-45% of dogs in the US are obese. Studies have shown that joint and locomotive problems increase by 57%, circulatory problems by 74%, respiratory problems by 52%, skin problems by 40% and cancer by 50% in animals that are overweight.

Large breed dogs that are overweight also are more prone to developing hip dysplasia. Obesity is especially dangerous for young puppies, as their underdeveloped frame cannot support the extra poundage that it must carry.''