Hi David,
Are you rendering with the “View: Mantra” option or “mantra1” when you click on the Render button in the viewport?
If you are not sure, then Right-Click on the Render button and select “mantra1” which will start a render. My guess is motion blur works for you now.

After you selected “mantra1”, it will be used for all future renders so just left click on the Render button in the viewport. Use Right-Click only when you want to use another ROP.
I hope the above helps!
Cheers!
steven