John Cazale was most likely not available on the day the cemetary scene was filmed.

No big deal in my opinion, at this point Fredo is not the main focus of the story.

Incidentally, in a past thread on this subject somebody (I forget who) claimed that Fredo CAN be seen somewhere near Connie if you look very quickly before they cut away to another shot. I'm not sure how accurate this is as I've never been able to catch a glimpse of him. Not that I tried more than maybe once...not really too important an issue in my opinion.

Apple