Sounds like you are doing everything right, could simply be that it's the second spawn.
Don't bother with the 3rd and 4th airstone untill well after the first week.
If this happens with the third or fourth spawn I would start looking for outside environmental causes, such as lighting or temperature fluctuations, mis-sized heater or leaking voltage.
Also, if you're dropping the salinity and there is ammonia present, even in small quantities, the pH difference between the tank water and the water you're using to drop the salinity could be turning the ammonia toxic, if that's the case definately look into an ammonia binding agent.
HTH
Dman