Danish aseels
Songster
Fertility issue may be do to things other than infertility. Egg handling or settings in incubator. I make no claims to my skills with an incubator as still learning, now from someone that hatches thousands annually. I might know something of value on incubator use in another decade. I just took my first run with a GQF Incubator. I still have trouble turning it off properly.
Some lines may be inbred in a manner that impacts fertility. It can also impact other attributes not related to fertility such as size. Hen I was gifted comes from a line that lost size causing breeder to loose interest and give them away. The recipient outcrossed them with Gowdy Japs to restore size and vitality at expense of gaining attributes not consistent with Aseels. Those birds were line-bred back to the undersized pure Aseel making for a 7/8 Aseel x 1/8 Gowdy Jap keeping larger size and getting back to more what Aseels are supposed to look like. My stag comes from a pure Aseel line where size was not lost although feathering is rather nice for an Aseel. His family was selected under a colder climate where feather selection might have operated stronger for resistance to cold than what those at lower latitudes endure.
Could you post pictures of your Aseels?