In your render settings, next to the checkbox for displacement, there is a little box for numbers. The numbers here must be higher than the number you use for the displacement setting in the material. The setting there may need to be very high, if you are dealing with split vertices, and the displacement will not carry evenly through because of those split vertices (at least, I'm pretty sure abot that last bit, double check with stewer or other smart folks)

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)