Training problem ~ yup! But you know we hear about a new case of this several times a year, and you’d think even cops would catch on that when somebody is preternaturally UNCONSCIOUS and not moving, that they are already down!
Without proper training, and given the truly weird effects of many street drugs, plus being amped on adrenaline themselves? Nope. Few people would.
That’s why proper training and regular refreshers are so very important. Yet a lot of departments and cities refuse to spring for it, then wonder why incidents of the type you mention happen.