Keep an eye on it
What it can look like. A general, vet-reviewed example — signs vary and this is not a diagnosis. When in doubt, ask your vet. 
Dull coat despite eating well in dogs
Clinical name: Poor body/coat condition
A dull, poor coat or weight despite a normal appetite.
Where it shows upgeneralized
Typical patternN/A.
Linked allergy typesFood allergy
Food vs environmentalCan accompany chronic food issues.
What you can do
Worth a vet check to rule out other causes.
Don't confuse it with
Other causes a vet will want to rule out: malabsorption, parasites, organ disease.
This is educational information, not a diagnosis or treatment plan. Always see your veterinarian about your dog's health.
Related signs
VomitingKeep an eye on it
Intermittent vomiting, sometimes after meals.
Diarrhea / loose stoolKeep an eye on it
Soft or loose stools, sometimes more often than usual.
More frequent stoolsTrack it
Going more than three times a day, sometimes with urgency.
Gas & gut noiseTrack it
Excess gas and audible gut gurgling.