D&C RARELY FAIL.
Medical abortion are more likely to fail.
D&C can fail because the abortion was done VERY early, say 4-5 weeks, and the doctor did not remove all of the pregnancy tissue.
He may have thought he removed everything, but he did not. Pregnancy that early is TOO tiny to see, so you go by volume of blood and materials removed.
SOMETIMES they remove the fetus, but they leave other tissue behind.