How close do you need to be to a lure to discover "lured pokemon"?
Based on watching this IGN video on how the lures work, it appears that you need to have your player ring intersect with the Pokestops rings to reap the benefit of the lure. If they aren't intersecting in any way, you won't find any lured Pokemon.
Since you said your ring didn't reach the Pokestops, I believe you were too far away.