I don't know either. It seems weird. He seems to spend more time interfering in chicken business than mating the hens. I wonder without competition a male muscovy gets lazy? My last male wouldn't mate hens his last year or two of life after I lost the other drake. He was old though. The hens would chase him around instead of him doing the chasing.I hope you can get a few to hatch some ducklings.. Here my drakes don't even start mating the females till 8-9 months old so maybe they just take longer to mature and be fertile. My females don't even start laying till 8-9 months old. I can't imagine why your drake at his young age wouldn't be fertile as the dickens.