Spell,
if you look at these functions you will see that they call the low power functions in the MAC.
the MAC will return not sucess in case the MAC does not allow you to go to sleep.
the typical scenario is that your device is a RxOnWhenIdle device (e.g. a coordinator or router)
and not a polling end device. in this case you will not be allowed to sleep.
Please look at the 802.15.4 reference manual for details about these functions, and take a look at the MyStarNetwork demo that utilized the power lib.
BR,
Mads