I'm playing through FR right now, and I have never had such a difficult time catching a Chansey in any Kanto region file, ever. I think I've seen 30 by now, and haven't caught one. I've been in the Safari Zone for days now, I'm going out of my mind!
EDIT: Immediately after posting, encountered a Chansey who fled the second I threw bait. *shakes fist at sky*