So I bought some of the food items the pets are requesting, and it shows up in my inventory, but every time I try to feed one it says I don't have what the pet wants. Huh?

Edit: and now they have all changed anyway. How often does it change?

Like, every minute or so? Maybe less than that. Some of the items rotate down, and then are gradually replaced from the top down. And sometimes all of them change at once. You have to be pretty quick, and refresh often. I refresh after every third feed or so.