Sim Won't Get Pregnant (Wonderful Whims)

I'm trying to get a sim in my rotation pregnant, but she hasn't gotten pregnant so far, even though the chance is set to 30% for both played and NPC sims and she's woohooed like 15 times. The issue is specific to her; other sims in the same save have no problem getting pregnant. I've had the same problem in the past with another sim. It's almost as though they were assigned the hypofertile attribute without me actually assigning it! (I checked; they didn't have it.)