Demographic Transition Model
As can be seen from the picture, as a country countinues in its DTM, it soon begins to lose a large majority of its population growth around stage 4. Germany is an older country, and has progressed throughout stage 1, 2, 3, and is between 4 and 5. Their citizens are starting to have at or below 2.5 children on average, and that equates to a low to declining population growth. Their birth rate is the main problem, because if you take a look at the graph, the death rate isn't changing and the birth rate is declining, driving down the population growth.