John Brown failed to achieve his goal because his goal was not something that an individual citizen could possibly accomplish. Brown wanted to end slavery and that was something that could only be done by the government as a whole. It was extremely unlikely that one citizen would be able to do anything that would bring about its end.
John Brown tried to bring down slavery, for example, through his raid on the armory at Harpers Ferry, Virginia. He planned to take the weapons there and use them to arm a slave uprising. To anyone who was being realistic, it would have been obvious that this could never work. Brown had no way to notify the slaves of what was going on. Even if they had all tried to come to Harpers Ferry, slaveholders could easily have stopped them. If they had gotten arms, the Army would have easily defeated them. This was a really silly plan and only a person who was somewhat delusional could ever have imagined it would have worked.
Brown failed to accomplish his goals, then, because his goals were simply impossible.