Make sure you're doing everything right, and practice I suppose? One stupid mistake I found myself doing was not holding down on the control stick throughout the move. Maybe you could identify similar problems?
Yeah, I am holding down the control stick, tapping the C-stick up(set to smash right?).
You don't hold "B" right, just hold down on control stick? Well, I tried it without holidng B and holding B on 1/4th speed, I can't seem to get it to work. Tried on all the speeds in training mode also.
Does even setting it on 1/4th speed actually make things easier? Because I know I am pushing the C-stick fast enough(I tried slowing the speed I tapped it to increasing the speed, but it still doesn't seem to work).
I can't even make Dimenion Cape last longer than it normally lasts.
Hmmm, does Tap Jump need to be set on(Jump when pressing up on the control stick)? I don't have that set to off.
I using the Classic Controller if that means anything.
* Shuttle Loop (Up+B): 9%, 12% on subsequent gliding attack
* Down Smash: 11+13% (on either side)
Also, neutral air is a good kill move at higher % (12+7% damage), and down air (7-9%) is great at gimping recoveries. Forward smash (14%) is a slower but nevertheless powerful alternative to the down smash.
Hope I've helped.
Anyway, that post did help, thanks.