Yes, it sounds like the eggs aren’t fertilized properly – I’ve had the same problem with my group of Xenotilapia bathyphila, they spawned and spawned but the females didn’t hold for more than 2 or 3 days.
Then I saw a spawning and noticed that the spawning male were very, very busy chasing other X. bathyphila males away – the other males were not actually disturbing the spawning pair, but the spawning male chased them away if they came nearer than 1 to 1.3 meter into his sight – it seemed quite unnecessary, and the result was that he barely had time to fertilize any eggs, before he went out on another chasing round. So, I tried splitting up the X. bathyphila group and placed each male with a couple of females in separate tanks. The result was quite good - the females hold the eggs and larvae as they should and produced nice batches of fry. Have you observed anything similar with a male being more interested in chasing other males than fertilize the eggs?
Your tank is very large, so it there should be plenty of room for pairs to find a quiet spot to spawn – I don’t know how you have decorated your tank, but maybe you could scatter some rocks on the bottom and in that way reduce the visibility between the fishes?
Is there anyone else that has tried solving “the unfertilized egg problem”?