The thumbnail image for one or several parts or subassemblies doesn't look right; they may be inaccurate due to a software glitch or contain artifacts (e.g. the grid or construction planes) that are ugly or make it hard to interpret the image.
To force Bommer to regenerate its thumbnails, you must clear the cache folder located in your Bommer data directory. For a guide on how to find this directory please click here; to clear the cache folder, simply delete all of the images inside the folder marked "thumbnails". Bommer will then recapture the images on the next export, and cache these images for future use.
To save export time, Bommer caches the thumbnails it exports. It will regenerate the images if it detects any kind of change to the component data (this does not include showing/hiding planes, grids, or other visual features within Fusion 360). If there isn't a change to the component, Bommer will assume the image has not changed and will use the one from its cache.