In QSP, impregnating NPCs are about 20% chance unless you have a specific spell called "Primova's Fertility" (earned through HOA MIA questline when finishing Wendy's content).
The impregnating of players also has a 20% chance unless fertile womb, but there are a few things that can prevent you from getting pregnant:
- Vines in your womb (Not sure if that was implemented)
- Having tentacles in you
- Taking contraceptives
- Already impregnated
- Being a beemorph (you have eggs instead)
If none of these then its possible its a bug.