I can only speak from personal experience here (so don't jump all over my shit saying "THAT'S NOT THE CASE FOR ME," it may not be, personal experience only):
In my younger days, I HAVE been with at least one guy who kept changing rhythm/position whatever ... I realize that sometimes the mouth/jaw gets tired but the thing is that 30 or so seconds when you're repositioning/revising/switching/unhinging your jaw some of us get seriously cooled down. You do that three or four times, it's shot for me, I'm overly sensitive and it's uncomfortable -- virtually sore, the female equivalent of blue balls, so to speak -- but I just ain't getting mine that evening (or at least for a while, I have to totally calm down and start back from ground zero). When you're not really, REALLY familiar with your partner (or younger, that can be the case too), it's tough to say, "uh, sorry, but it ain't happening."
I've also had the situation where he just can't seem to hit the sweet spot -- he goes everywhere BUT there -- which can be nice for a bit but you try to be subtle and get him where the right place is and he's like HOMED IN on the wrong place ... or what he's doing just DOESN'T feel right, maybe it started out right and then it's like, "PUUULLLEEESSE stop touching there! Stop touching it LIKE that!"
It gets to the point where it's more frustrating than fulfilling, but you don't say that to your partner, because ... well, you don't say it to him because he's not in the ZONE THAT night, doesn't mean he won't be batting 1000 the next night, if that makes sense. You make a guy too self conscious about the whole oral sex thing and there's the fear you might be waving bye-bye to tongue action permanently. You don't want to encourage him in bad habits, but you don't want to discourage the enthusiasm, either
And honestly, those of your who say, "well, just tell us what we're doing wrong ..." umm, again, personal observations here:
a) It generally doesn't make any difference, he may change what he's doing for a couple minutes, but he pretty quickly lapses back into his "comfort zone."
b) I haven YET to have met a man who enjoys receiving either directions or constructive criticism.
