Here are the solutions that I can remember, and the one that cured my problem with non hatching eggs. The same is also true for deaths of one species of shrimp in a multiple colony environment.
1. pH fine tuning.
Tiny adjustment in acidity/alkalinity can sometimes make all the difference. This is what cured my M. kulsiense young hatching problems. It only took an adjustment of ~.25 to get healthy young.
2. Test and Removal of excess toxins.
It sounds like you've checked this. But look for test kits for the more unusual things like iron, copper, calcium, salinity/gravity, etc.
3. Time.
This is the most frustrating. Sometimes waiting will allow the tank to settle, and fix the problem. This is particularly true if you've done multiple remedies, with no appreciable results. Rather than going from one cure to another, time is needed to determine if the treatment worked, or not.
4. Separating the affected group to another tank. By doing so, you run less risk of adversely affecting the other inhabitants of the original tank.
I hope you find your solution!
