I've been using this for a few years, I used Lumigan off-label before Latisse came out (they are the same thing made by the same company).
I've learned that there is a growth cycle with Latisse. With daily use your lashes will continue to grow longer and thicker until about the 8 week point, then you'll notice there is a shedding period when you longest lashes start to fall out (noticed especially when using a lash curler or applying mascara). During this shedding time if you look closely at your lash line in a mirror you'll see that there are new lashes growing in. If you continue with the daily applications of Latisse the next generation of lashes will grow as long as the first.
Unfortunately this cycle is the dirty little secret of Latisse... it does in fact make your lashes grow longer and thicker, but they don't stay in that perfect state forever! They shed and renew just like normal lashes.
If you notice you longest lashes are starting to shed don't despair, just keep using the product and be patient...they will grow long again!