dogs not eating/skinny dogs

Question
So I have four dogs.  One is doing fine.  He is at normal weight and eats.  The other three do not eat all of their food and look skinny.  The three that don't eat sleep in the house (the one that looks fine sleeps outside till we can get him a crate.  Is there a reason they might not be eating all their food?  Could smell be a factor?

All are males

The one outside, who looks completly fine, is neutered (purebred aussie shepherd shorthair)

One inside is a complete mutt of medium size. neutered

One is a pomeranian who is neutered

and the other is a chiweenie (daschund x chihuahua)and unaltered

I feed them Nature's Recipe  

Answer
It's possible they aren't eating all of their food because they just aren't hungry, but if they haven't been tested for intestinal parasites recently I would definitely do that to rule out the possibility of any problems there.  If they do have any, getting them dewormed will hopefully improved their appetite and their weight.

Amanda