Younger males may not be 'ready' to breed, though they may go through the motions. If you already received two infertile clutches from this female after being paired off with the male, you really only have two options. One, put them together again and let them breed a few more times. This way the third clutch may have fertile eggs. Two: keep them separate and try again next year, maybe pair him off with a different female as well.
Another option is to have your vet do a sperm count on the male. Though rare, it is possible he is infertile.
Some dragons may need a cooling period as well (brumation). Not sure if you brumated the male and female or just put them together. Though in most cases, brumation isn't required to produce fertile clutches, some dragons may need it.
Only other possibilities I can think of is the eggs are fertile but are going bad for another reason. If they are not shelled sufficiently, the genetics of the offspring are not viable (hidden negative traits/mutations may be hidden in these two dragons and when combined, produce a lethal result, thus babies die in the shell before any major development). Candling the eggs should show veins though. Infertile eggs won't have veins.
As you haven't really indicated what makes you think these eggs are infertile, not much else i can guess on why they failed to hatch. Do they just look bad? Or did the collapse/mold in the incubator or just never hatched after incubating for over 75 days?
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PHLdyPayne