group.move() weird pose of detached object

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Hi!

I am currently working on a pick and place application, using move_group interface. After bringing the gripper around the collision object, i attach it to the end effector:

group.attachObject(collision_object.id);

Then picking it another place, i detach it:

group.detachObject(collision_object.id);

When i detach it, the box stays with one edge on the groud, one in the air. How could i fix this? Please help me, Thanks