I find the thumbnail is slow to update and often doesn’t update for days (at all?). A work around has been to choose “Restore default thumbnail” and then “Set as thumbnail” on the desired frame. The thumbnail has become an important feature to me and I use it to convey information to my team. The inconsistent thumbnail updates are a little problematic.
Suggested solution: an option in the right-click menu for the file to “Refresh thumbnail”.
Or just a more consistently working thumbnails system…
Well, there is that option too. 
Until this is live, you can force refresh your thumbnail by removing and re-setting the frame as the thumbnail. 
That forces immediate changes.
That usually works but I have had times it didn’t.
Same problem, and none of these solutions are working. I’d love a more reliable way to update the thumbnail
Slightly related: I have a stubborn file that won’t show any thumbnail at all, even if I set a frame as thumbnail and then reset it again. I’ve tried duplicating and making edits to the file, but it’s still just gray! 
Possible solution:
Since the file’s frame that contains the cover/thumbnail is presented as a “special” frame once you set it as a thumbnail, maybe ONLY this frame should have a refresh button beside it’s custom icon:
Same here. I have a large file that just won’t accept new thumbnails and always just shows the grey rectangle.
The thumbnail updates every time for me now… maybe it was a silent fix?
We have a lot of projects on the go at the same time and the thumbnail has become a crucial way for me to keep up with the status of each project. An automatically updating thumbnail feature would be highly beneficial to me and my team!
Yep, I noticed that too 
Just tried to set a thumbnail for the first time and noticed that it never updated. The only thing that worked was quitting (desktop app) and launching again which then showed the thumbnail. But, I do not accept that should be necessary to just refresh a thumbnail.
In the help doc, the steps shown to create one concludes with “Figma will update the file’s thumbnail and show the custom thumbnail icon next to the frame.”
But technically it doesn’t say when that will happen 
Same thing for me. I tried multiple times to do the two-step, Restore, Set as Thumbnail, moved the Cover to the first page, and nothing worked except quitting the desktop app and restarting. Sheesh.
I used to be able to restore the thumbnail and then select my artboard again but it doesn’t appear to work 90% of the time…
Please fix this Figma team!
+1: This just doesn’t seem to work reliably (or at all?). The thumbnails in Figma should update in real-time, for starters; it only updates for me if I quit the app and restart which seems impractical. When I searched for guidance to get a better working thumbnail, Figma recommended using “1920x960” which also does not work (crops the content). Then I noticed a “safe zone” recommendation within that Figma post of “1600x960” – but this ALSO does not work (still crops the sides). This has added up to a lot of work to get something so basic working – and it’s still not even displaying what I want without cropping. What’s the magic proportion to get a thumbnail to display without cropping my content? I tried copying and pasting the Playground thumbnail and it still didn’t work for me.
I use these for file status, etc so very important that it updates quickly. For me the delay is too long and that trick works to reset the thumb but kind of annoying.
Experiencing this issue as well. Thumbnail not updating. It seems to grab the canvas color of the top/first page page but vectors from other pages 
Same problem here. It’s not just annoying it’s crucial for a work environment that focuses on “collaborative working”, that changes made by a team member are instantly visible to other team members. We already had problems in the past because of thumbnails not updating. Please fix this asap.
+1
Very annoying issue and inconsistent interaction patterns. Sometime it works, sometims it won’t work the whole week. If a more consistent auto refresh is not an option, manual refresh would be very appreciated
