The round nosed lead bullet is just not reliable. It will penetrate deeply some times, not penetrate at all other times.
Years ago, when Memphis Police carried the Model 10 S&W loaded with .38 Special with the 158 gr. RN bullet, a Memphis police officer was shot with his own revolver. The entrance wound was just below his nipple on the left side, the exit wound in the middle of his back near the left shoulder blade. Penetration?
None. The bullet struck between two ribs, skidded along in the groove between the ribs and exited in his back. A flesh wound.