Running robot_pose_ekf and amcl

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One possibility is that there are interfering published transforms. Did you try dumping the tf graph both with and without amcl running? E.g., rosrun tf view_frames. To debug this, you could try spawning the amcl with modified odom_frame_id, base_frame_id, and global_frame_id parameters. Confer http://wiki.ros.org/amcl#Parameters (near the bottom of the page).

You wrote that robot_pose_ekf is required by leg_detector. However, there is no indication of this in the package description. It is required to specify a reference frame (parameter fixed_frame), which could be provided by multiple packages, e.g., robot_pose_ekf or amcl.