This sounds like something that would require investigation by a veterinarian. Perhaps, there's some nausea setting in or some other digestive problem. I don't know why it wouldn't happen in the morning, but my older dog with stomach motility problems and acid reflux always struggled more in the evening compared to the morning meal. I suppose the fact that dogs are more active in the morning (excited to start the day, more likely to move, walk around) may play a role since often in the evening they are tired and more likely to lie down, but this is just speculation. Please report the behavior to your vet for diagnosis and treatment.