Wish I knew the cause of this, I wouldn't have lost a few female angels. Sometimes epsom salt helps, 2-3 teaspoons per 10 gallons long term, 1 tablespoon per gallon for 5-10 minutes, keep an eye out for signs of distress with the short term bath. Another thing to try is bringing up the tank temperature to increase metabolism, and don't feed for a month. The extended starvation period hopefully will cause the body of the fish to absorb the eggs. The higher temperature increases metabolism.
This has about 50% success with angels, as far as cause I wish I knew. I do know a diet too low in protein will cause eggs to be not quite as sticky, falling from the surface they were spawned on, perhaps this has something to do with it, maybe not.
