I ended up with Castle and Knight against just a Castle in a recent endgame and couldn't close out the win. I tried against computer several times afterwards but it seems to me that unless your opponent makes a mistake and loses the Castle there's no way to force mate here? Am I wrong? I'm rated 1300 at the moment so hoping some of the bigger brains out there have an answer on this one.
You are right. It's commonly a draw unless the minority king is locked on an edge. The similar endgame R+B vs R is also commonly a draw but it is more dangerous for the minority side. Even GMs lost it after mistakes!