Here are a couple of graphs that highlight what is happening.
This shows how much Jehovah's Witnesses are lagging behind population growth.
The second graph shows that the Watchtower is reporting some growth over a 20 year period, whereas the census is not.
As SBF points out, there are a number of reasons. One is the ability to report less time (ie 15min per month). The other is that inactive ones are less likely to still believe it is the truth and claim to be JWs on the census, due to the growth of information on the internet over that time.