In Regards to Child Credits

1- For the child income credit of $300 per child for the next few months, I only received $600, but I have 3 kids (3rd one born in March 2021). How do I update to have them send me the full amount I qualify for?

 

2- Can I add my daughter born March this year to my 2020 taxes and how to do so.

 

3- My tax rejects repeatedly because of earned income credit they said they are missing form 8812, I keep uploading it and it keeps rejecting for the same reason, I submitted about 15 times and running out of hope.