Final answer:
A drive stun is sometimes not very effective due to the difficulties in maintaining contact, the lack of sufficient contact points, and the challenge of achieving a pressure point application.
Step-by-step explanation:
A drive stun is sometimes not very effective because all of the above reasons: difficulty in maintaining contact with a combative suspect, insufficient contact points for NMI (Neuromuscular Incapacitation), and the challenge of achieving a pressure point application on a combative subject.