I have a 3DOF robot arm and all of them are rotating on the same axis (say z axis in urdf model). Inside RViz with MoveIt's motion planning, I am able to drag the end-effector around and perform the motion planning with your trac_ik_plugin as the kinematic solver.
However, when I write my own ROS package to call the API or use MoveGroup class from MoveIt, it never returns a solution to exactly the same target end-effector pose. I have no such problem when working with a 7DOF arm with exactly the same setup procedure (except of course the arm is designed differently).
So my question is whether trac_ik is designed to work with small number of DOF (e.g. 3DOF in my case)? Or there is any tested examples or tricks to get it working?