Examine the parameters:

Increase the maximum number of ITL slots for segments on which a blocking user performs a transaction. 

Decrease the SESSIONS_PER_USER limit in the profiles assigned to blocking users. 

Set a limit in the proles of blocking users to control the number of data blocks that can be accessed in a session. 

Use Database Resource Manager to automatically kill the sessions that are idle and are blocking other sessions. 

Decrease the IDLE_TIME resource limit in the profiles assigned to blocking users.