I have let a few kids go, over the years, who would not settle in. In fact, between 2012 and 2013 I let three kids go for this reason. It was a bad year for screamers. And, NO, I am not talking about normal transitional crying for a few weeks. I'm referring to SCREAMING for 7 or 8 weeks that got to the point of utter ridiculousness. And for EVERY one of those kids I termed the parents were doing stupid shit at home that was not conducive to setting their child up to be successful in group care.
It is RARELY the kid but the parent that is the problem.