I am playing on 1.12.2. I can marry villagers and able to procreate with the one I marry myself and marry them with a matchmaker's ring just fine but they never procreate by themselves. My village of maybe 10 villagers is now down to 3 villagers. I married 2 of them with the matchmaker's rings and they don't do anything. I even added more doors and buildings to the village but still no offspring.

In other villages, I noticed that many villagers wonder far away from their village for no reason. Probably because they have no set home point.

You must force them to procreate with each other by gifting them cake.