Calling FltSendMessage from an arbitrary context

When I call FltSendMessage from within a function that is called from FltMgr, that is to say with FltMgr on the stack, the function works as expected and I get the message in the user mode part.

However if I call FltSendMessage from a different context the funcion returns SUCCESS but the code in the user mode part does not receive anything.

Is there any restriction on the calling context for a FltSendMessage ?

I am working under VISTA/32.

Inaki.