Yeah cax that's pretty common. Maybe try more pushing down, and left, and right, forcefully at the base with your lower hand, rather than just pulling straight out with it, .. you'll get a different dynamic, less slippage, and more stressor on the base. Remember what DLD said about always digging back in for another low grip.
(If you use two hands at the base as I'd described on page one or two you'll get a
radically better grip and high force for the base that barely slips. After a slight motion, one had runs into the other and they pretty much stay right there until you swap or dig back in.... but all of that is a -completely- different exercise and if it addresses girth at all, on its own, it's almost accidentally. But I , like mentally , suspect that it would make a great superset exercise to the
SRT-SB, particularly if you're having too much slippage, which is why I think I keep mentioning it. When I do the Size Blasters, I do those too .. whatever they're called, and I can tell ass is being kicked).
Another bit there depending somewhat upon how your body happens to have been made: By varying the amount of either grip force [with the thumb] or downward force [with the wrist/arm] across the top of the penis at the dorsal vein(s) -- picture featuring the thumb flatly as a bar against the top, instead as of part of a circle that's evenly everywhere -- .. you can change your erection percentage around a little ... versus just what the amount of sinusoidal relaxation, arterial dilatation, venous compression (blah blah blah) itself is doing. People will tend to grip kindof randomly everywhere, or on the sides, when it's a couple of tiers of veins on the the top that do the heavy outflow.
With the
precise amount of erection percentage, and a strong grip, you should be able to get a compressed column to form between the two hands (one at the glans, one at the base) while the lower hand is still pretty low towards the base. This will give you more to grab against, .. and, as it slips, it should then be slower .. and also form the girth component of the exercise.
I'm finding that I like to do at least some of the reps with the top hand pronated (pinky facing away from the body). You can't pull as hard with the top hand, but the upward motion of the base hand can go up further before running into anything .. and this allows a full-length 'reverse
SSJ' that feels like Pow!