This is only a brief answer.  There can be many reasons.  Please ask yourself the following questions: 

1)  Have I ever participated in an occult activity? 

2)  Do I have an occult or abominable object in my home? (see      Deu. 18:10-13)

3)  Do I hold unforgiveness, resentment or bitterness towards      anyone or towards God?

4)  Am I in rebellion against authority?  (parents, employer,      pastor, etc.)

5)  Am I living in sin – practicing sin?

6)  Do I hold attitudes of prejudice or anti-semitism?

7)  Do I dishonor my parents?

8)  Am I legalistic?  Do I try to “add any extra condition for      achieving righteousness” other than what God’s Word says?

9)  Do I talk in line with what God’s Word says?  Do I speak well      of others or do I gossip and slander others?  Do I complain a      lot? Do I speak fearful or worry-filled words on a regular basis?

10)  Do I watch “dark” movies or play “dark” computer games or        listen to “dark” music? 

If you answer “yes,” confess your sin to the Lord, ask His forgiveness and then take action to rid yourself of ungodly associations, movies, music and/or objects.  Renounce any involvement with evil.  Ask the Lord to cleanse you with His Blood (1 John 1:9).  Attend a Bible-teaching church, read the Word of God daily, pray and fellowship with strong, committed Christians.  Seek counsel from your Pastor.