Without looking at the plane, it's hard to say. Do they both start up at the same rate, with one stopping sooner? Or does one move more slowly than the other? If the former, then flexing could be a problem. With the pushrod(s) disconnected from the servo, you should be able to raise and lower each elevator easily by hand. If not, look for a binding problem. If both move easily, but one moves faster than the other, make sure each pushrod is connected to the same hole on the elevator horns (i.e, if there's four holes in each horn, both clevises have to be conected to the same hole on each horn).