why the arm become awkward in moveit once it was mounted on a move base?

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Now I try to use turtlebot_arm mounted on turtlebot to do some sim. But I find that once it was mounted on move base. I can't plan the arm easily in moveit demos only if I use approx IK solutions. But I don't want this because I know it will have bad effect on future sim and the approx IK solution seems it doesn't have an SDK in python. I have tried tuetlebot_arm and youbot and they have same problems. can anybody solve this question? thank you