Interesting, are you dropping it over the blue area? One of my playtesters told me that was a little confusing for them.
Had a go on desktop and no such problem. It really was my laptops touch pad. I think it might interpret me pressing any button on the keyboard as releasing the mouse button for some reason.
Now that I actually finished it I would like to commend you for making such generous box collision physics. Positioning them a little under a platform pushes them back and I don't have to restart!