Peter, I've seen the moons' shadows in my 8" GSO dob. Questions/suggestions - are you well collimated? Do you see some banding on the planet's disc? Has the seeing been satisfactory each time you have tried? Is it too bright? Are you up to as high a magnification as seems reasonable - perhaps 200x or more?
Try your moon filter if it is too bright for your eyes.
Know where the shadow will be from your software so you know where to look.
If seeing is not great, the shadow will come and go in your view.
Just watch continuously as you track the dob.
If you are not seeing at least one band across the planet, I suspect you won't see the shadows and you might wish to check your collimation.
Good luck! Tell us how it goes.
Eric
