Working fine here. Have you checked that this feature is actually enabled? You can check this from Device Manager > System devices > HDD Protection – Shock Event Driver > Tap settings.
You can also try double-tapping to wake - don't need to use as much force then. The force and timing involved is quite similar to that used for double-tap on recent Nokia phones (for example, to display the clock when sleeping). The accelerometer inside the case is also used for hard disk protection (say, when you drop your Booklet). Hope this helps. - Miles.