I don't think the mods have something to do with that. It's just a question of a lucky moment for multiples to come.

I only use baby boost when it's needed - I mean, after a doctor's appointment. I tried to use once (without the prescription) and nothing happened. In fact, they had problems having kids after that.

What I noticed is that through the generations it is getting more difficult for them to have babies. Why this is happening I truly do not know.
For example, on my 1st family they had babies every time they tried. On my second they had problems, and now - on my 8th - I have to keep trying and trying for the babies to come.