Dunno, really.
Only got the impression it could have to do with sex position. Nearly everytime I set them in either missionary or cowgirl they got pregnant. Futas, girls, servant or dungeon keeper, didn't matter.
Other than that, about 50% of the time somebody cums, the partner gets pregnant, and even some of the servants just free-roaming the dungeon (set to no job) did some shenanigans on their own and some got pregnant that way. With fertile trait, 99% of sex action got them preggo.
But that is just very subjective feeling on my part, and mind you I played this game for about 3 weeks before I found something a bit less repetitive & more engaging over on the f95 page.
Iffen you wanna breed some more furry things, with better graphics & less strain on the GPU, try Breeders of the Nephelym (no spiders in that one though^^). Kept me for nights over a couple weeks, pretty funny game, really. And a lot more polished & closer to being finished than this one.
Hope you get your spider family going anyway, sounds like a nice, cribbly-crawly idea.