How good should inverse kinematics with KDL or Trac_ik work?

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Hello List,

I have a basic 5DOF arm that basically works with moveit! and inverse kinematics. I adapted the moveit_ik_demo.py script from the ROS by Example book and use phrases like:

  # Set the target pose.                                                                                                        
    target_pose = PoseStamped()
    target_pose.header.frame_id = reference_frame
    target_pose.header.stamp = rospy.Time.now()
    target_pose.pose.position.x = -0.274
    target_pose.pose.position.y = -0.019
    target_pose.pose.position.z = 0.306
    target_pose.pose.orientation.x = 0.057
    target_pose.pose.orientation.y = 0.047
    target_pose.pose.orientation.z = 0.99
    target_pose.pose.orientation.w =  0.113                                                                                                                
    right_arm.set_pose_target(target_pose, end_effector_link)
    traj = right_arm.plan()
    right_arm.execute(traj)

But the problem is that if I give perfectly reachable poses in the set_pose_target function, on average only 5 out of 6 calls results in success. The others result in

LBKPIECE1: Unable to sample any valid states for goal tree

Why does it fail so often?

I use a Core2 quad CPU so I would have thought that 5 seconds should be long enough to find the solution. This is both with KDL and Trac_ik. Am I missing something?

As said, the poses are perfectly reachable, on average 5 out of 6 times the solution is found.

UPDATE: Solved. The positions I used were not perfectly reachable because I left out some decimals in copying the values to the test program. See some comments with the answers below.

Thanks in advance, Sietse