Rotating the stick only helps to ESCAPE technically, when you are on the offensive side of a submission you can either mash the 4 attack buttons or rotate the stick, it's no difference. And since my hand starts to hurt when I rotate the stick for a bit, I usually mash the buttons when I'm on the attack.
And yeah, it's really hard to sub guys with good sub defense. When I was playing D.P. the other day he chose Micke Swick with a sub defense of ca. 60 and I had no problem subbing him with my 92 sub offense. In our second fight when I used Big Nog and he was Frank Mir I didn't even come close to subbing him.
What I usually do to sub guys with good sub defense is mount them and (try to) block all their transitions. When I feel that they might have gassed a bit (I play without stamina meter) I just go for the armbar or triangle choke and mash the attack buttons like a bitch.
Wow, I really need another word that is synonymous to "sub".