Adjustments not reflected in preview thumbnails

While processing Nikon NEF files (D300), sometimes the adjustments I make don't show up in the thumbnail though the actual file is fine when I click on it. It seems somewhat arbitrary - for instance, when I apply a color correction to a folder of 200 files, it makes the adjustment to 30 of them, and then the rest don't change - or maybe the last bunch changes and 10 in the middle change but all the others don't. I first assumed I'd begun making adjustments before all the settings & preview files were generated but I've been very careful to make sure they're all there before making adjustments. Must be related to that somehow - like I'm moving on to the next adjustment before the previous one is complete but it's so random & intermittant, it's very hard to tell.

Any suggestions?
Thanks.