Platform can only jump when on the floor.
Due the very rough collision polygons in that circle, the platform is constantly in the state of 'falling'.
Hence, not on the floor.
You have to make the collision polygons of that circle a LOT better so it comes a LOT closer to a circle.
Instead of being just a rough polygon.
Now, i am to lazy to do that. (that is like making at least 240 points, and moving them in the right position)
And you did not even try.
So, lets force the system to make a perfect polygon that is close to a perfect circle as possible.
You need a plugin in for this, it will tell you which one on loading.