No Appetite

The past few days, my dogs seem to lose their appetite. We feed them twice a day with dog food but they seemed to lose appetite. So we tried feeding them with table food, and they ate a lot. To give them nutrition, we sort of give them alternate diet, dog food and table food. Now they got tired of the diet again!

Can anyone give me a suggestion on what to give them? They like bread but I know it's not nutritious to just feed them on bread alone. What is the most effective vitamins to give them, ours do not seem to work.

Any suggestions please?