After loading the back on my longarm, if I have a side that sags, I unroll the back and the leaders (leaders are important) and reroll being very careful to keep as even as possible on the sides. This almost always works to take the sag out. Someone gave me this tip when I got my machine 4 years ago but unfortunately I do not remember who to give credit to.

my only customer quilt that this problem occurred was one that she had 3 pieces---one large one that was on Length of grain, 2 that were WOF--and one of those was cut from wide back fabric, but really square--so there was lots of bias stretch in one piece of fabric--just could not avoid having a small tuck on back

Is it obvious from the start, before quilting I mean, so that maybe they back can be re-done? I've only loaded a practice quilt so far, so haven't run into this.

I think it could be all of the above. I know if you sew those pieces together and they are not done well you will get a sag, lots of people gets sags with just a regular back if not put on properly or not squared. Too many things to pin point it.