This is because your villagers are eating more of this type of food than you can produce at the moment. If you look at the detailed rations information you should see how much food is eaten per day and how much food is made per day.

Since your villagers eat all food types in equal amounts you need to make enough of each food type to have it count towards your food bonus and for it to show in the granary.   For example if your villagers  are eating 9000 food a day and you are feeding them 3 types of food you will need to have 3000 of each made per day minimum for it to show up.