Problems when adding subscriber to openni_tracker

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Hi,

I customized openni_tracker to fit the needs of an application. Basically I just needed the part to detect new users and lost users and the coordinates of some joints, so I removed all the unnecessary tf parts and just publish the coordinates i needed. So far no problem, I encounter problem when I tried to add a subscriber. By default openni_tracker doesn't call ros::spinOnce so I added it just before r:sleep() (r being ros::rate) so that the subscriber callback would be called but when I do so the coordinates are always set to 0, 0, 0 like openni can not get any coordinates. Can anyone tell me what may be the problem? Thank you in advance