I would pick a piece id which has been logged as failed audit and search for it in the logs.
Then you can check if something had been logged about that file. Like what happened when it got uploaded and maybe some other additional log messages.
Second I would check in the storage folder if the piece is truly not there.
From the satellite id you can get the storage blobs folder: Satellite info (Address, ID, Blobs folder, Hex).
The first 2 characters of the piece id is the name of the folder. Check the storage and the trash folder if the piece is there.
If it has been successfully uploaded and it’s not there, then you have a problem. And if it is there, you have another problem.
Maybe it is a file system problem and you need to run a file system check and repair.