I've had this happen with one of my second gen mothers. She kept getting illnesses, which I immediately cured. They have all four food groups and over 1.5K food constantly available with a full meal provided whenever I am actively playing. They have the bath upgrade, hammock and air purifier with several other home upgrades, no trash, weeds, etc. Her fed bar is full, but happiness is a bit lower than other family members who stay at elated. Her health is at half or just below. It is not related to her energy level at all, as it began in the morning when her energy was at it's highest point.
My theory is she may have come down with an illness just after I shut down for the night and it must have decreased her health over several hours time. I am hoping that continued praising of "healthy" behaviors (working out, quick work out, relaxing, etc.) as well as providing fruit, vitamins, and regular meals will allow it to improve over time. As luck would have it, the only item in the store was the antibacterial hand wash, which didn't seem to help as no family member was sick at the time.