Hi,
Do you render through render manager or locally?
Do you have any dynamic geometry in the scene, like Hair, displacement, proxies …
Did you try to set a simple V-Ray material in the Override material slot?
If the suggestions above doesn’t make any difference you can try to debug the scene geometry by deleting some parts of it, one by one.
Note that you can always send us the scene file or stripped down version to support@chaosgroup.com and we will try to help.