Sometimes the melee creep dies before making it to Walker either to fireballs, other creeps, or something else, and the ranged creeps don't stand in range to activate the Walker's vulnerability.
I've had two games today where I pushed creeps all the way up the lane (Yellow lane for me) and they have stood at the Walker for 10-20 seconds doing no damage to it.
48034849
The above is the game ID from the most recent time it happened.
There is something going on with that ledge above it I swear. Minions will get stuck. My solution to this issue is just running in as seven with fleetfoot and shooting it. I feel like this is a pretty big issue.