Generally slugs are due to the male not being fertile/not having enough sperm to fertilize the eggs in the female. The second one would be if he had been bred to numerous females and just didnt have enough rest between breedings. I have had one female first season ever for her she threw 5 slugs and nothing else and the very next year gave me 8 perfect eggs from a pastel breeding, she was normal, and all eggs hatched out as pastels. But the rule, and i use that term loosely, is to not breed a decent sized male, about 750 grams and up, to more than 5 females. I have had smaller males breed up to 7 females and do just fine but some times they just dont have the goods to get the job done ya know. Sorry for how this is all jumbled up but I have been up for over 24 hrs and im half asleep lol.