You can only walk your dog once every 25 minutes. Before walking your dog, wait 25 minutes with the game on, in sleep mode, or off. Also, the dog won’t be very active during the day if it stayed awake all night.
To walk your dog(s) without having to wait , take your dog for a walk and run into as many "?" boxes as possible. Then, take your dog to the park and change your accessory. Once the game auto-saves, turn off the Nintendo DS then turn it back on. You will be at your house and have all the items you collected, as well as be able to take your dog out for another walk. Repeat to get lots of items quickly and build up your dog's stamina.