Exactly. I had a hen and rooster shipped from back east a couple of years ago, and they were routed through Arizona and took three days to get here. When I took them out of the box, the rooster drank so much so fast that he aspirated, and gave himself pneumonia (I know that's what it was because I watched him do it) he almost died. The vet put him on a whopping dose of Baytril, and he has been sterile ever since. But we still have him, and love him no matter what, it's our Duncan....Sounds as if you have covered all your bases. I have had some duds myself with chickens but never ducks but then again my brother was the real duck expert not me.
It is very hard to say what causes infertility but I know treatments and past infections can cause infertility. I know a woman who had a bout with a nasty respitory illness in her chickens and once fertile hens and roosters became duds after treatment.
I would try a new drake possibly one that is between 8 to 12 months and throw the dud drakes in with a different hen and see if he produces before culling them. Good luck.