“He said to them in reply, “An evil and unfaithful generation seeks a sign, but no sign will be given it except the sign of Jonah the prophet.”” (Matthew 12:39)
  
Jesus did not give a sign to the scribes and Pharisees, because they did not have the heart to repent.

In fact, Jesus cured a man who had a withered hand on Sabbath, they went out and conspired to put Jesus to death. (see Matthew 12:9-14)

Not only were they lacking a heart of repentance but were also completely blinded by jealous, “Then they brought to him a demoniac who was blind and mute. He cured the mute person so that he could speak and see. All the crowd was astounded, and said, “Could this perhaps be the Son of David?” But when the Pharisees heard this, they said, “This man drives out demons only by the power of Beelzebul, the prince of demons.”” (Matthew 12:22-24)
  
They faced the truth but spoke out words not of their conscious as this opposed the Holy Spirit.

Let us listen to Jesus’ severe testimony against them, “Therefore, I say to you, every sin and blasphemy will be forgiven people, but blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.” (Matthew 12:31-32)

Jesus also added, “I tell you, on the day of judgment people will render an account for every careless word they speak. By your words you will be acquitted, and by your words you will be condemned.” (Matthew 12:36-37)

Jesus is merciful; He cannot help us if we do not possess a sincere heart.