Part of their decline is in the makeup of the congregations.
At one time Kingdom Halls were filled with every age group. My hall had about 50 youths (13 - 20 years old) during the late 1980's, but this doesn't exist anywhere anymore.
Today's Kingdom Hall is full of the ageing and aged, a few born-ins now in their 30's (often single women), and the occasional single mother and her children.
Also, the increase comes mostly from within. WT figures show that it is taking longer and longer to bring in new members. This trend, along with the dieing off of old ones, is not healthy or sustainable.