He failed due to whatever happened ten years ago with Harry. We don't even have a good theory yet, IMO, of what that was (and the canon options are misleading).

Apart from that - a day before that - he had not failed at all. His old-style abusive tactics were keeping the Death Eaters in line and were successfully terrorizing the populace, and he was winning the war using those tactics.

However, those tactics may be inappropriate to his current position as Quirrel, because he doesn't have any real minions or subordinates, just a few people he manipulates without their knowledge.