Ok everyone... so I have been hearing a lot of complaints about samus's grapple failing... There are 2 reasons why her grapple will fail... 1. You have used the grapple 3 times off the stage before touching the ground, on the 4th time, the grapple will fail. The second reason is if you use the grapple too high above this stage or on the plane horizontal to the edge/stage, or the plane perfectly vertical to the ledge. If you do not understand this, I have added a pretty picture diagram to help you understand.
http://i.imgur.com/YjKZsJ0.jpg
http://i.imgur.com/YjKZsJ0.jpg