My personal solution to solve this problem is:
1. Check the credentials
ssh -T git@github.com
The name printed out should be your wanted user name.
If not, go step 2.
2. Connect ssh.
cd ~/.ssh
3. Connect to wanted ssh
ssh-add ~/.ssh/id_rsa_account1
4. Check the connected ssh list
ssh-add -l
If there shows your wanted ssh. Then problem should be solved.