IK succeed for constraint-aware but fail for non-aware

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i'm following this tutorial http://ros.org/wiki/pr2_kinematics/Tutorials/Tutorial%204 to do the inverse kinematics for my robot arm. but for the same goal position, the constraint_aware_ik succeed to find a solution while the non constraint aware ik cannot. after dig into the source code, i find that the constraint_aware_ik ultimately call the function searchPositionIK() and the non-aware ik call the getPositionIK(). my kinematics solver is the KDLArmKinematicsPlugin. is there anybody encountering the same problem as me?