DisplayMaster problem

I am using the IDV classes to build navigated displays. I have a MapProjectionDisplay (in 2Din3D mode) and add some displayables. I then call the draw method. I have also attached a DisplayListener to the MPD.

In offscreen mode (pass offscreen == true in the MPD.getInstance() static method, I never see a FRAME_DONE DisplayEvent arrive into my DisplayListener. Further, the call to MPD.getImage(boolean) (derived from DisplayMaster) always hangs. I suspect that the latter is a consequence of the former.

So my question is, are there further calls I should be making in my code to ensure that offscreen rendering works with MapProjectionDisplay objects? When I was doing the same work 'by hand' i.e. using just visad objects, the getImage() call did return.

I am running a 1.5 vm, with java3d version 1.5 also, on RHEL4.  Driver is 
'fglrc' from ati, if that matters.

Any help appreciated

Stuart