Some people were recently complaining that some classic achievements weren't unlocking But then they'd unlock on their own some hours later. I don't recall if flawless was one of them, but it's worth a shot seeing if it pops back up. Besides that, there are STUPID caveats to that achievement that don't make sense. Stuff [i]like[/i] a teammate joining or leaving mid raid will cause it to fail. I see you already tired the cache clearing. So beyond wait-and-see or Run-it-again, there's not much else to try