Recently I faced an issue, a reseller is unable to switch accounts from his main cpanel account. He can switch to some accounts without any issue. However getting Login attempt failed screen when trying with some accounts. I have checked the ownership and all but all seems fine. Googling doesn;t help much.Atlast I found what the issue is.
Check if the user is present in
/var/cpanel/suspended
if the user is listed there move it to somewhere else.
vi /var/cpanel/users/username
check for the entry suspend=1 if its there comment or delete it.
Goto the reseller cpanel and try to switch account, it will work. Also try the same if you have encountered an issue with account suspension.