Seems like they respond to one call one, year, and another the next.... Most calls are fairly proven. I use a hyperhot cow call, with decent response, but overall do the best with a single reed diaphram call. Practice with one and you can do any number of variations of cow talk, bugle, grunt, squeel, etc.... Any decent hose works good for grunting/bugling and in the off season you can talk turkey and call predators with it as well.

Get a couple of different calls and practice with them, then on those days when the elk are especially fickle, you may find one that gets a response. Definately include a diaphram call in the group.