I failed to catch Snorlax the first time I met him in Route 7. Then after finishing the game, I was happen to pass by a sleeping Snorlax again in Route 7 (on the same bridge), but once again I failed to catch it.
How often do Snorlax appear on that bridge?
More info: Ever since I posted this question, I've been coming back to the bridge almost daily for a few weeks, but I haven't seen him again. So in total I've only seen Snorlax on that bridge twice, and I'm thinking that there's some sort of trigger that causes me to see him the second time.