From what I seen in the video, the character slides only on the edge of the platform, if that is correct then it seems there is a collision polygon problem because the platform looks a little lit sloped near the edges, sadly if you are using a tiled background you cannot change the collision so you will probably need to fill the edges to be exact like the rest of the platform, if its a sprite then you can check the collision polygon and adjust it a bit on the edge.
I had a similar problem on my latest platform game I am working on, I had a sloped tile but the player was not sliding on it because it was a tile background and the collision was a square instead if a triangle, I fixed it by changing the image to sprite so I can adjust its collision and after that it worked fine at a cost at slight performance change.