Yup, unfortunately it does happen. Your penis has clearly become conditioned to PESG's routine. You may want to focus on linear progress which is normally what results in gains. Much like weightlifting....slow progression with added weight/reps is what leads to bigger strength and size gains. Perhaps you could stick with your current routine but maybe add some reps every other workout which will allow your penis to slowly adapt to a harder, more demanding workout. If you haven't responded at all to this routine, it is definitely not intense enough.
*diesel