[CentOS] Centos6: how to debug suspend?

I have a desktop installation with Centos 6. A lot works fine, but
suspend to ram or disk does not work from the KDE menu.
If I choose suspend to RAM, the screen goes dark (black-light on) but
nothing further happens. I can give my password to remove the screenlock
and continue. But the machine stays on. Obviously the suspend has failed.
If I use pm-suspend (as root) all works fine. I've looked in
/var/log/messages but there is no hint there. The question is thus, how
can I debug this? Could it be a permission issue? Do I need to add
myself to a specific group?

Thanks,
Theo