Even the young kids that left or got in trouble came back in their late 20s and 30s.
Because of psychological coercion. "I am married with kids now and my kids are asking about grandma and grandpa and uncle or aunt so and so....I'd better go back to the toof" BTW the only real growth in westernized nations is primarily through kids growing up.
Some from immigration but thats also slowing down somewhat. Many immigrants here in the U.S. often view the JWs as a social club to enjoy and promote their own culture while away from home. Even these people are waking up.