I know this question is old but I can help you. You need to have your cat "full" and "quenched" and leave the DS for 40-50 mins. Do not even walk in front of it otherwise the facial recognition will recognize you. Then come back to it and tap the "whistle" button. Your cat should come running in with a gift.
This may not be helpful, but one reason may be that you need to keep the pet clean, quenched, and full. You also have to pet it at some times, or it will get a bit lonely. Try taking it walking or going to the park, too.
