I've had people not know why I'm reaching out; I've also gotten references selected by the candidate that did not have good things to say. eg "X is difficult to work with."

And most people aren't good impromptu liars. So pushing a bit with a reference on what did you work on together? Why is this person fantastic? Would you hire this person? can get you far. And if the reference has left their shared employment, the classic: why haven't you hired this person?