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The Watchtower covers itself for every contengency. If they show an increase it is because of Jehovah's blessing. If they show a decrease it is because of Satan and they quote the scripture about the road to life is narrow....
Now they have a new tactic. Although they have greatly increased their presence in third world countries, they have not had the expected success. So their explanation is that although the meek respond well, they are not expecting a mass exodus out of non-christian nations. So if we do not see significant numbers to account for the billions of hours spent in field service it is understandable. I suppose this is in addition to the invisible growth that we also do not see.
They also stress that we should not give up preaching in those areas.