It happens more often than you think. There are many reasons as to why. The wrong time of mating, which can be helped next time by ovulation testing by your vets, to something wrong with the dog's sperm, or the bitch getting an infection. These are just 3 reasons.
You have the opposite problem to me, in that my bitch has a season every 19 weeks

which means that she doesn't have enough hormones to stay pregnant. My male is also infertile, not a good combination ;-)
Next time your bitch is in season I would take her to the vets for ovulation testing, and make sure that the male has sired at least one litter and is, therefore, fertile :-p