Privileges are set up when a user first accesses the PostgreSQL databases page in their cPanel. If the user doesn't visit it, phppgadmin is not going to work. Also, ensure the user does not have a space in their cPanel password, causing issues logging into psql/phpPGadmin.
Sometimes, when getting a Login Failed error in PhpPgAdmin, the reason for this is the server preserving a cached user and password for the account in question.
The solution for this is moving the pw.cache file to the side and then running upcp --force.
Renaming the Current pw.cache files:
mv /var/cpanel/pw.cache /var/cpanel/"pw.cache-"`date +"%Y-%m-%d"`
Forcing the update to UPCP, which will rebuild the pw.cache to the current cPanel user infomration.