ritalin stopped working

Hello. I'm 29 year old mother and I'm pretty worried for my little son’s health. You see, he is 7 year old and since he was five, he has been diagnosed with this behavior disorder called ADHD.
He was acting strange and didn’t have any friends. He was hitting and biting everyone around him all the time and I've decided to take him to see a child psychiatrist.
He has determined that he suffers from this ADHD disorder and prescribed him some Ritalin. He was using him for these 2 years constantly and his behavior was improved significantly. But, in last two months I have noticed that this Ritalin definitely stopped working because all of his symptoms have returned. Why has ritalin stopped working for my son?

Hello. I have some experience with all this because my brother has also been diagnosed with this behavior disorder few years back.
The good thing about this is that it was just a temporary thing and now he’s completely cured.
He was first prescribed with the same drug like your son- Ritalin. The same thing happened. The drug worked for him for a couple of months and then- it stopped. When we asked his psychiatrist about this- he told us that this it is very common thing for a patient to become immune on drug and that in that situation he should be prescribed with the new one. So he prescribed him some Adderal, which he used for a year or two and then all his problems disappeared.