Ok, I had this ridiculous problem today. I wanted to run Dolphin as root and I typed "sudo dolphin". Didn't work. I typed "su" and then "dolphin". Again to no avail. So little googling told me that I'm doing it wrongly.
is the command. And it works :)
Maybe I'm just very unexperienced, but I think that last time I typed "sudo dolphin" it actually worked. Don't know why that changed.