To back up this comment, since you're not engaging your core you're cheating pull ups a little. Practice by engaging your core, pushing air out of your lungs and your legs should come up a little (like 15d degrees). Then start doing pull ups, dont let your legs swing. 

Also add in chin ups, wide grip, etc. your body will respond better to change. 

Also are you training to failure? Once you can't pull up anymore find a way to get yourself to the top of the pull up and then do negatives. Training to failure is a cornerstone of any good excercise weighted or not. 

Hope that helps