Your question doesn't make sense ?  Who said it wasn't filed ?  Did you get an IRS CP2000 for missing income ? 

Did you fail to enter it on the return ?   Did you file without it and then go back later to add it but failed to mail in an amended return ? 

There is no such thing as "additional earned income."    Are you confusing that with additional child tax credit?  If you got the child tax credit it is on line 13a of your Form 1040.  If you got the additional child tax credit it is on line 18b of Form 1040.  Not everyone qualifies for it.  What did it say as you were preparing your return?