Gosh, that one threw me for a bit of a loop <img src="smileys/smiley36.gif" border="0" align="middle" />
I haven't had much dealings with Construct's platform system unfortunately, but I've narrowed it down to being some sort of problem with when the sprite is standing on a platform. The sprite will stab as needed when he's in the air, however when his feet are on the ground something is apparently overriding his stab animation.
What I ended up doing is removing the "Stopped" tag from the Default animation of the sprite. That seems to fix the stab problem but he will no longer automatically revert to his standing animation when he isn't moving. I then included the event Sprite is on ground + Sprite is not moving + Animation "walking" is playing --> Set animation to "Default"
That seems to make it work for the most part.