My dog does not eat his food when I feed him at the beginning?

[TiiMoo]

I have a Chihuahua mixed with Fox Terrier. He is 3 years old in human years. In the past when I fed him the food, he would eat it right away and finish it quickly. I bought a slow feed bowl to him because he did not chew his food when he was small. I have been using this bowl more than half year, and it's slowing him down to eat well. Recently, he starts not to eat his food when I feed him right away. After I put his food on the floor, he went to smell it a while, and just left it there. So I just left the bowl, and sometimes he would start eating and finish it eventually. I think it's because he is very hungry. I'm worry that he changes his eating habit suddenly is having some health issues.

Thanks.