If you added something to the actor after initially putting him there, try deleting him from the scene and put him back in (sometimes actor behaviors in scenes do not update if they have been customized).
Or add a block saying "if y of actor + height of actor > y of platform" then always "set y of actor to y of platform - height of actor"
If all else fails, put a boolean for when a collision happens and draw or print it to show you whether the collision is actually occurring.