[suse-security] su - problem

I'll try to describe my prob in details: I log in as normal user xyz. I want to do somehting, I can only do as root, so I type "su". Prompt: Password: I enter my password. Answer: login incorrect. This happens always, everywhere. Who can help me. I've checked all the files that harden_suse changes (according to log-file)... Couldn't find anything. Any help? thx