I got 8 slugs from a 2305 gram female ball python this morning.

Infertile is generally the male.

Slugs are generally a female issue....not mature enough, or your temps are too hot. We've had 164 eggs out so far this season. No slugs, and only 1 infertile egg. Not bad results.

I drop my hot spot to 85F during breeding season and just raised it to 90F, as I've already had 38 females ovulate.

I have had 3 different virgin females lay over the past 2 years and no slugs , all fertile, 1 boob egg that hatched out healthy. They were bred to proven males, except for the 2010 female who bred to a virgin male. The lowest weight of the females was 1300 gms but she was a 2010 virgin.

I've also seen it suggested that slugs can happen when a female is too fatty. Probably something about the over-abundance of fat, probably visceral fat rather than subcutaneous if I had to guess, influencing the animal's hormones, like in a mammal.

The Barkers' observation (and I tend to think they're right on this one) is that a slug is a follicle ovulated prematurely. Before it was developed enough to then become a viable egg. Realistically, any number of things could trigger premature ovulation of some or all of the follicles.

On infertile eggs . . . Some might not be the male's fault. Because the eggs develop so much while still in the mom, I suspect a few of the eggs we call infertile were, in fact, fertilized, but died prior to laying.