The vol pot used to have a center detent, then they switched to a pot w/o a detent with the first sets of 2010 ES', and since last year I've heard they're back with the center detent. So one guess is perhaps different sourcing for their pots?
You can easily check for any friction with the knob: the rubber knob simply pulls right off. Be careful and pull gently/slowly straight up. Once out maybe you can see if anything looks awry. Then reinstall knob, making sure it's not jammed all the way down so as to bind at the body? At least it's something you can check w/o damaging anything.
Edward