sudo doesn't work but su does

Yeah. OK. I'm an idiot. I can't type. Please ignore.

I should have typed

sudo ls

not

su ls

And then entered my administrator password. When I do that, it works. D'Oh!

Hi,

I've enabled root and set a password. I know what the password is, and it works with su root. I am using an administrator account to login with a complex password (that I also know). However, no matter how carefully I enter the password, sudo always fails as follows:

$ su ls

Password:

su: Sorry

I can get around this with su root, but it's risky and I'd rather not. Any help gratefully received.

Thanks