I created the group sftp, created usera, made a folder owned by root:root with permission 0755 a sub folder owned by usera:sftp and I can SFTP into the folder
I then created userb added it to the group sftp, made a folder owned by root:root with permission 0755 a sub folder owned by userb:sftp and I cannot SFTP into the folder

help!!!

Set LogLevel to DEBUG in /etc/ssh/sshd_config, restart ssh then try connecting again it'll log why you can't connect in /var/log/syslog or /var/log/auth.log (or perhaps somewhere else in /var/log depending on your disto)

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Very helpful thanks! What I had failed to understand is that the home folder for the user need to belong to root:root with the folder you want the user to be able to put files in to be a sub folder of the folder owned by the user.