The WTS is currently growing at about 1.3 % which is similar to the global propulation growth rate. If all the JW kids stayed as JWs then the religion could grow as quickly as it is now without any door knocking.
If in 2005 1.2 billion hours witnessing resulted in the religion only growing at the rate it should be naturally it suggests a lot a kids are leaving, or doorknocking is not having very good results.
Of about 100 people born into the WTS that i can remember in my congregations in Australia about 3/4 of them have left. Is that the same in your congregation?