I'm sorry, I can't give you an answer about why the internal hardness of fish belly appears during cooking. But you can ensure that the cooked fish belly tastes soft and waxy through reasonable material selection and correct production steps.

In the selection of materials, we should choose fish with rich meat and elastic taste, such as cod and shark. In addition, the quality of fish maw is also very important. Choose high-quality fish maw without peculiar smell and residue.

In the process of cooking fish belly, you need to pay attention to the following points:

1. Before cooking the fish maw, soak the fish maw in water for a period of time to remove some salt and peculiar smell.

2. When cooking the fish belly, add seasonings such as ginger slices and onions to increase the taste and taste.

3. Cook the fish belly for enough time to ensure that the fish belly is completely cooked and cooked.

4. In the process of cooking the fish belly, stir with a spoon from time to time to avoid the fish belly sticking to the bottom of the pot.

5. The cooked fish maw should be cooled in time, so as to avoid overcooked fish maw caused by residual heat.

In addition, if you have trouble cooking fish maw, you can try stewing it in a pressure cooker. Pressure cooker can shorten cooking time and better preserve the original flavor of ingredients.

In a word, you need to pay attention to the selection of materials and cooking methods if you want to make soft and glutinous fish bellies. I hope these tips can help you.