Do you think this might have something to do with lack of nutritional balance as compared to what rays might ingest in the wild?..
Although Shrimp and Seafoods are high in protein and great staple diets for rays, perhaps the crustacea they eat in the wild is the more optimal provider of nutrition for pregnancy.. Charvet-Almedia's studies on both feeding and reproduction of potamotrygon hinted at these aspects, but provided no comparison to hobbyist work/husbandry..
I have never seen a pic of a wild ray 'gorging' themselves like we feed them in captivity.. perhaps rays do this type of feeding in captivity because they actually have to overeat food that is less nutritionally balanced in order to intake adequate amounts of varied nutrients..
Sorry for your loss Freddy.. If I get some blacks down the road we will have to swap genetics..