There have been a lot of "fixes" for this.
Read the link below and do a search and see if some of those options will work for you.
I have tried a lot of them (if not all of them, but I can't be sure). And some of them work...........until they don't.
I don't understand the problem with why some parts don't show a preview and some do, but it is definitely an intermittent problem that keeps coming back for me.
Sometimes, I will see nothing for the preview, sometimes, I will see a Solidworks icon for the preview, sometimes, I will see the part/assembly/drawing, but zoomed way out or way in, and sometimes it works perfectly. Sometimes, I will see ALL these things on the same part in the same day.
At the least, make sure this is checked: