Actually, they are getting pregnant -- that is where all the little ones come from.
It is that most of the grown ones are dying off, and encounters between male and female become far and few in between.
The young do not have enough time to grow up and have their own before the spread of the virus ends.
The infection rate must be slowed down more!! But not too much :+)
I think you should check the age of the Persons left after the virus has run its course. If they are all around the age of, somewhere about, 5500 or under, then they are too young to procreate (see my previous note, 14 hours previously).