No, but i can gamble.
Character is animated and the origin is dancing from frame to frame, or the collision polygons are dancing from frame to frame, or the frames all have a different size causing origins and polygons dancing from frame to frame.
As a result the characters looks like it is on the floor, but in reality it is falling on certain animation frames. Can not jump if there is no floor under Character.
If my wild gamble is right, then you need to fix the animation (hard to do once messed up), or use a 'helper object', and 'Pin' the animation to it as a 'mask on a face'.