There were times that I would flick up on the keyboard to use a word prediction and nothing would happen. I do think BB should offer some kind of sensitivity level controller for this, but the problem was sometimes caused by my own fingers!
There are times I still had a thumb on the keyboard while simultaneously trying to flick up for a predicted word. Obviously, this interferes with the gesture. There are just some times you are typing so fast, you just don't realize how you're already prepping to depress the next key.
I'm placing this here just in case you might have wondered why the gesture might not be working for you sometimes.