WINCE Power management

I have implemented the power management for USB HCD driver.
I have implemented the IOCTL_POWER_CAPABILITIES, IOCTL_POWER_GET and the
IOCTL_POWER_SET in the driver.
When I start the system the device is enumerating through the
and also i am returning true from the IOCTL_POWER_CAPABILITIES when
success.My problem is ... I am not able to call the IOCTL_POWER_SET from my
application always.
I have used the SetDevicePower API from an application and the call
succeeded to my IOCTL_POWER_SET once. But the next time onwards I am not
getting a call to my IOCTL_POWER_SET portion in the driver. I tried using the
DevicePowerNotify also. These APIs return 0(SUCCESS) but i am not getting a
call in the driver. COuld any body please help me on this?
Please let me know how to pass call to the IOCTL_POWER_SET from the
application successfully when ever needed