I'm really interested in this answer.
A lot of snakes turn out to be misgendered, I get that since popping proves male, not female, so a female could easily turn out to be a male.
But very rarely have I heard people saying their snake actually cannot breed or has an issue breeding. Sure there can be issues with slugs, but sometimes those same snakes will produce good eggs the next year.
Sometimes I have heard people saying their male is reluctant to breed, but usually that comes up early in a breeding season and not at the end of it, to which I assume the snake just wasn't ready for breeding at the time and needs more time.
Other times people breed young or underweight and have issues with them not breeding.
Of 3rd winter females, second winter males, both of adequate size and in correct conditions, I'd really wonder how many snakes have been duds.