
There is different reasons for single pups, wrong timing of mating,stud not very fertile,bitch not very fertile,infection,genetic abnormalities cause reabsorbtion, one pup dies and the remainder in that horn are affected by the decomposing pup which may be too far on for reabsorbtion just to name a few.
Some of those may be repeated next time or it could be plain sailing.
Last year I had a bitch scanned at day39 and 3 pups were alive, she went into labour 2 days early and had 2 partially decomposed pups and a stillbirth which I knew was alive the night before because I heard the heart beat, if I hadn't had the scan and the 2 pups had been reabsorbed then I would have thought she only had a singleton pregnancy.