Yes, your lack of gains was because you weren't stretching at the appropriate tension. When using the
SG, start with a lesser force during your first set. Just stretch half inch above your erect length.
After your first 1 hour or 1.5 hour session, the second to last, you start increasing the tension on the rod every 15 to 20 mins. You will eventually increase it to max tension before 1 hour.
Always follow this pattern and trust me, you will gain 0.1 with the
Size doctor or
SG or any extender at all every month.