I've seen slugs in clutches from all the top BP breeders in the US. Which tells me that no one really has an idea of any things we can change husbandry wise to reduce the chances and numbers of slugs.

The female could have a hard time retaining viable sperm long enough, the male could have low fertility, the timing of breeding could be off.... lots of stuff.

One thing I have seen is it seems much more likely to get slugs when breeding a male to multiple females. And in all reptiles I've worked with obese animals always have lower fertility.