Just now Ive been in a second time(not something I normally do) , and for some reason I just decided to check one nest again and when I counted the chicks there was one missing

Will the parents accept it back?
Baby is about 10 days old, why toss now? Parents both defend the nest.
If it does get tossed again, should I try putting back again or try putting in another nest where the eggs are due to hatch on 11th?
The crop was full so could the parents have been feeding it on the ground?
Could juvenile gouldians have anything to do with this?
Diane