The biggest difference between the two boys is that one of Mantoo's birth parents is still in the picture. Mantoo's mother is nowhere to be seen, and his father doesn't want to have anything to do with him. This makes the process of Mantoo's adoption all the more complicated, and it's no surprise that it takes the Brierleys two years to adopt him.
During this time, Mantoo is subject to sexual and physical abuse, something that thankfully doesn't happen to Saroo. Inevitably, such appalling treatment takes its toll on Mantoo, and it comes as no surprise to seem him acting up when he finally arrives in Tasmania to stay with his new adoptive family. Mantoo's clearly been traumatized by his experiences back in India, and it will take him much longer than Saroo to adapt to his new surroundings.