White balance not working after update to 15.2.0

today I updated C1 subscription to 15.2.0. I opened a job I shot on the previous version  and the white balance tool has stopped working. When I click on a white patch, the colour value in the WB text window changes appropriately but the preview screen remains unchanged. I have tried a another shoot from a few weeks ago and the WB appears to work fine. So the problem appears to be with this shoot only. Does anyone have any advice?