People who are not losing weight are simply eating too much. It really is that simple. It's the opposite of course of why they gained weight in the first place.
I'm familiar with the concept of starvation mode and what the data show is that it more or less doesn't exist. Intuitively we know this must be true as well, since starvation is real and those on too few calories do continue to waste away until they die. The metabolism slows under decreased calories, but if you cut substantially below maintenance only when you lose a substantial amount of mass will the metabolism ever match that new normal (because you have less weight that needs to be maintained).