Turret Press Help Videos available: http://leeprecision.com/Turret-Press-Help.html
The most common cause for the Three hole Turret press to fail to index is an over tight clamp screw. This collapses the clamp (the clamp is the two die cast parts that fit around the end of the ram) so that the outside diameter of the hex ratchet drags on the inside diameter of the clamp. The fix is to loosen the clamp screw until the clamp halves are just together. Attempt to index the press which will probably fail at this point, tighten the clamp slightly about 1/8 turn and try again. The idea is to sneak up on this from the loose side as opposed to try and loosen to the proper point.
Four hole Turret presses that have index problems are handled differently. With the turret resting in the incorrect position, place a 1/4" wrench on the flats of the actuator rod and hold it steady. Do not use a pliers - this will probably damage the surface causing further problems. Then, with the other hand, operate the turret manually until the turret is in the correct position. This should be done with dies in the turret to ensure that the turret indexes correctly under the drag of the dies in place.
With both style presses, lubricate the outside diameter of the Turret with 30 wt. motor oil, or something similar.