This sounds like the old 'returns null' issue with skeleton tracking that many people have had with this feature since RealSense launched in 2014. Skeleton tracking mode has to be enabled, otherwise skeleton joints return '0'.
This link may be of use to you.
Skeleton tracking is part of the Person Tracking feature. Officially SR300 is meant to support Person Tracking, according to the documentation. In reality though, Person Tracking is rarely used with SR300.
The only SR300 Person Tracking video that I know of is this one from Mike Taulty (who has also produced tutorials on his blog ( https://mtaulty.com/ ) that have been very useful for other aspects of RealSense programming).