Are you dropping the people in the family in the right place? Maybe not enough initiating/training on cooking meals and possibly not the right kinds of foods? Are you buying all 4 food groups? I haven't tried it, but if there isn't a variety of food to prepare meals with, all they will be able to do is snack.

Even though mine eat on their own, I still pull someone over everyday at dinnertime to make dinner and pull the whole family to the table to make sure that bar is 100% full, not just part way.

I don't know if this is the case with you at all, but from what I can tell, it looks like many people think that once a character or family does something a few times there is no more training to be done on that activity, but I don't think that's true. Like real life, people can fall out of habit and so can our families.

When you initiate a meal, drop them on the table/bar, not at the counter; that's where the process for preparing a meal is triggered.
Try using candy as the praising tool when you get the opportunity.
Make sure all 4 food groups are available, not just a lot of one.

Hope this helps a little, and keep us posted!