Its rough when you loose an animal for unknown reasons especially when everything seems to be done correctly. Dont beat yourself up about it, as others have said it does happen and is the potential downside of breeding.
I would carefully watch the females that you lost males to in the future to see if its a particular snake or another factor. Keeping meticulous records and time will hopefully help solve or eliminate the problem. I am also curious to see what others have experienced in breeding related losses of both males and females.