Driver unloads - do not know why

Hi,

My minimal driver (Usb device driver with UMDF) unloads before the framework
calls the OnD0Entry-method. I used the umdfSkeleton example from the WinDDK
help as starting point.
Also I return the correct interface (IPnpCallback) when the framework
requests it via QueryInterface.
In every function that is called by the framework I return S_OK, so there
shouldn't be a problem.
I have no idea, what I'm doing wrong. Also I can't get any information on
why the Host-Process might cancel the driver load.
I hope someone can point me some advise on how to get mor information about
that error.

Thanks