Being that it is used, the previous owner may have changed the springs.
From the factory it is supposed to ship with a 16lb recoil and 21lb main spring.
Springs are easy to change, at least the recoil spring is. The mainspring will take a bit of mechanical aptitude.
Try cocking the hammer first before racking the slide like another poster said. If it is still difficult, I would swap the recoil spring with a known 16lb recoil spring.