When DKIM is not working sometimes , it means the encryption of the body itself is not working good if your DNS are working properly and are tested OK.

Just look at /var/log/mail.log

If you have an error saying that breaking line are more than X characters with an error on it, you just need to raise this limit doing that :

postconf -e ‘smtp_line_length_limit = 20000’

Put a big value…here i put 20000 but you can try to put what you want under this value

and after do not forget to restart postfix :

systemctl restart postfix

In Debian (adapt to your OS)