On average, I witness 95-100% of my (ready to breed) females lock up.
Of those, roughly 60-80% of my (ready to breed) females will go on to ovulate.
Of those, roughly 5% of the eggs laid will be slugs.
The key to all of this is: ready to breed.
If the female isn't ready to breed, she won't breed and she won't ovulate. If she did happen by some weird twist of fate to do those, she will most likely give you all slugs.