If you enable signature checking, you need to have the most recent key file imported. It basically was a race condition, Richard didn't have your new gpg key imported yet.
I suspected that something like this was the case but I could not find a verbosity option that would tell me what key is was trying to find. Adding -vv to gpg added loads of output but none of it was very helpful.
How did you discover it was Rons key that was missing?