Howdy,
Were you using it wet?
I would suggest removing the wheel from the grinder, and soaking it for an hour or so. Then put it back into the machine and try again.
The uneven surface should not matter if you properly use the guide for the tool. For a while you may get a pulsating contact with the surface your are grinding, but these wheels abrade rather quickly and I would expect that the high spots would even out in a few minutes of use.
HTH,