This happens in all species, including foxes. A study in Norway was done on 47 vixens, both young and old, which did not produce. Seventy percent had implantation zones which meant fetuses died early in life. Others which had normal litters had 20 percent more implantation zones that live fetuses.
The causes in foxes are not understood, but in other species include viral infections, hormonal imbalances, and stress.