As I see, in Mark, Matthew and Luke, the great tribulation is related to the destruction of Jerusalem, and Matthew says that " immediately after the tribulation of those days...... shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming on the clouds of heaven with power and great glory........... Verily I say unto you, This generation shall not pass away, till all these things be accomplished. "
So, if Jesus said what Matthew wrote, the prophecy of Jesus failed, because during that generation, the tribes of the earth did not see Jesus coming on the clouds as a consequence of the destruction of Jerusalem. That generation passed without seeing Christ for the second time.