Ren couldn’t pull the lightsaber to himself cause it flew to Rey.
Why was it flying to her? Because she somehow managed to beat him when they were both trying to pull it. Even if she initially pulled it only by herself, he should be easily strong enough to pull it back to him while it was flying. Remember, this is the guy who stopped a blaster bolt in mid air.
Ahh why am I even trying to argue over this stupid thing.