There are various recurring problems with the previews or thumbnails of Things:
Previews of STL files are often not generated, not even after hours or days. There seems no rhyme or reason in this, no relation to model complexity. It is just random. This seems to come in waves: this issue will exist for days to weeks, then it will magically appear solved, but a week later it could be back. Currently (2020/03/04) many users are complaining about this. Please fix this for good.
Thumbnail or gallery-sized preview images are sometimes not generated for uploaded photos. The thumbnail or preview shows the grey gears image. A workaround is to completely delete the photos and upload them again, obviously a hassle. This also comes in waves.
Sometimes the blue preview image for STL files is generated, but there is no Thingiview button to enter the 3D preview.
Sometimes the thumbnail shown in Thing listings does not match the first photo in the Thing's gallery. The thumbnail must always match the first image.
There are various recurring problems with the previews or thumbnails of Things: