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If I understand correctly what you want is the robot to actually "hit an obstacle" to push it? If that is the case the problem you are experiencing might be wheel slippage. Specially if you are using the simulator.

If the object that you are trying to move is significantly heavier than the Trutlebot or the friction of the object with the floor is too high or the friction of the Turtlebot wheels with the floor is too low, then the wheels will slip and that will cause the localization estimation of the robot to be incorrect. If the slippage is not too big then AMCL might help.

You can confirm the "wheel slippage" theory by using RVIZ to visualize your TF tree, you should notice that when you start Gazebo the /map and /odom frames will be perfectly aligned and will be almost perfectly aligned as the robot moves, but after the robot starts pushing an object the /map and /odom frames will diverge pretty quickly.

I hope this helps