She is very underweight, so the vet told me that she had to eat dog food all day. However, she only eats in the afternoon, regardless of whether it is human food. Never, but never, eats in the morning. The vet told me to leave her without food in the afternoon so that the next day she would wake up hungry. I did that, but the next day she didn't eat in the morning either, only in the afternoon. The vet had also recommended that I mix a can of wet dog food with the dry food for her to eat, but all she does is lick the wet food out of the dry one. Is it absolutely necessary for her to eat in the morning for her to gain weight? How can I solve my problem?
The only thing I can recommend for you is establishing a new habit.
It would be ideal if you took a walk with her in the morning or did some other activity every day at (roughly) the same time. Now, start giving her a small amount of food after that activity. Wait 10 - 15 minutes for her to start eating, but don't stare at her the whole time or she might be too uncomfortable to eat. If she doesn't start eating after that time, you simply take the food away and give it to her in the afternoon with the rest of her food.
Repeat that every day, even if she doesn't eat. One day, she might start eating a small amount, but it might take several days for her to be in the mood. Once she does eat the small amount given, you can sneak in a slightly bigger portion and see if she eats it all.
You should always take the food away that she hasn't eaten. Moderate eaters tend to lose their appetite if the same old food is always available. But if the food suddenly isn't there anymore and she cannot eat at any given time of the day, the food becomes more valuable when it is available and must be eaten before it disappears again.