Windows explorer not refreshing

Hello,Windows explorer not refreshing. In folders where video files are, there were thumbnails (small picture of the video clip). Now there are only medium icons with "mp4" text inside.Once when I opened such a folder, there was a yellow progress bar running at the upper side and slowly the icons became thumbnails.In "view options" I cleared file explorer history - no change.The video files are ok, they are not new, they all play well in the player.Ran sfc scan, no corrupted items.Not updated anything.Your help will be appreciated.ThanksMotim