Sorry to hear the news.
What did her uterus look like during the section? That can tell you a lot. Also, did the vet do a necropsy? My guess is that the placenta detached, but without ample stimulation, your bitch didn't go into labor, causing fetal death.
If you decide to breed her again, I would first have her screened for Thrombopathia, if you haven't already. Bleeding disorders can cause reabsorption of fetuses and fetal death.
If she's clear there, use a progesterone ovulation test kit to maximize fertilization. Confirm pregnancy at 30 days via ultrasound, then x-ray a week before due date. This will tell you whether she is reabsorbing puppies and what the actual number and position of pups are.
If she only has one ot two, schedule a section no sooner than the day before final due date. If she has a normal size litter, give her a chance to whelp. Contrary to popular belief, a bitch who has had a section can whelp normally on her next litter.
Give it some thought and better luck next time. I've had my share of breeding problems the few years and it sure does get depressing.