Movements are "rapidly multiplying" or exponential. This multiplication means that, at the minimum, they must double on a regular basis. Time to Double is a measure of the speed of any kind of viral fad. The "Rule of 72" is a quick and easy way to figure out how long this doubling will take: divide 72 by the annual growth rate of the movement, and you'll get the time in years.
The challenge a lot of movements and fads face: as the movement grows in size, the speed of doubling degrades.
Reasons for this vary by organization, fad or movement. Generally, it seems to me to harken back to the difference between early adopters and later adopters: early adopters tend to be "sneezers" who spread a fad far and wide, while later adopters are more likely to "use" or "participate" but less likely to spread.
Movements institutionalize. There's more being a church member, less being an evangelist. There's more using an iPhone and less being an iPhone evangelist.
When movements run up against the wall of a language or culture, it's exceptionally hard to cross the cultural boundary, and this is really where time to double drops dramatically.
The bottom line: in most instances, it is easier to start a new movement (send out new movement planters/starters) than it is to rev up the growth of an existing, large movement.
If you find the speed of growth is dropping, you can look at your training programs and see whether people are being trained to share their faith and make disciples. But you have to recognize only a small percentage will actually do this - and that percentage will decline in time. It may be better to look at how your church/movement is sending out new movement starters.