What speed are you doing when you pull back? Do you think it's most likely a plane thing, a pilot thing or a joystick thing?
As long as you're doing 55kts or so, a 172 should pretty much take off by itself. If you think it's an aircraft thing, make sure the trim is neutral, the mixture is rich, the throttle is fully open and the flaps are up or on the first notch. Under those circumstances you should be able to accelerate to 55kts in a few seconds, holding the stick neutral as you go, and then a gentle twitch should unstick you from the ground. If you pull back too hard you might actually be in a stall and mushing forward, rather than gliding gently up into the sky, so make sure you're not overdoing it and trying to drag the thing off the ground in panic. If the end of the runway seems scarily near, just close your eyes! :-)
But if you are going plenty quickly enough and there's no response from the elevator it might be worth looking at the tail from the outside to check it's actually moving up far enough - maybe your joystick driver has some kind of limits set? X-plane itself doesn't have such a setting that I know of, so you haven't missed it there - it'll be in the s/w control panel for the stick.