This is probably happening because the Platform Object moves too fast and some times, between two ticks, it travels more than the width of the solid objects, so it never collides and it looks like it passes trough.
This is most likely to happen on Firefox, because it has some terrible performance spikes lately, that seems to affect collision detection (among other things). Try previewing the layout in Chrome or NW.js, I believe that you'll get different, more as expected, results there.
You could also try to slow down the platform speed a bit, or use the clamp expression to limit the Sprite's position on the X axis. The event for doing this should look like this:
System|Every tick-----> s1 | Set X to clamp(self.X, "lowX", "highX")
where "lowX" and "highX" should be the number values that define the furthest to the left/right that you want your Sprite to be able to move.