How does the animal system in the home work?

I know if you affinity is high enough you get an exclusive accessory for themm bit the thing is all my animals are at full hearts since 2 months and they still dom't give me anything. i'm grooming them every day, give them food everyday and nothing. latley I decided to just change some animals and the new one gave me it instantly but the first animal I ever invited still didn't give it to me. what am I doing wrong?