If it's the Tessar lens, then it will not be sharp until about f/11 and higher towards the corners of the 6x9 negative. You'll find that it behaves like a 6x7 when opened up and then a 6x9 when stopped down in terms of corner-to-corner sharpness. I've dealt a lot with these cameras as a backpacking companion and can say that was a very common experience with many different brands and lenses.

Outside of advancing the film in the methods aforementioned, also be absolutely certain that the lens and shutter is parallel to the film. I've seen a many that it ever so slightly isn't completely parallel and makes both ends slightly out of focus.

Lastly, if it is a problem with the RF, they're quite simple and I could email you instructions on that. I've taken apart and rebuilt close to ten 6x9 folders, including that model.

Best bet: Tessar lens design.

That's just my $.02