Weird problem! I oraganize each portrait shoot into its own folder, which has 2 subfolders: one for the processed Photoshop file, the other for the RAW files, which is locked. My user account on Big Sur became corrupted; Apple Support had me generate a new user account, copy my files to an external drive, then copy them back into the new account. When I opened LR, I had no previews! Thinking it was a corrupt previews cache, I moved the previews file out of the LR folder. LR immediately started generating previews - EXCEPT for RAW files more recent than about mid 2019 - all the old files generated previews! Pointing LR to the folder with the unprevied RAW files did not help. Could there be some weird security protocol preventing preview generation in these files? Please help!