To do a true pull up the scapula needs to be in the right position, and your lower trap, biceps, and lats will work much better. With the scapula out of its position, you are asking your teres major and minor and only a portion of your lat to do most of the work to lift your entire body. This might be one of the reasons you cannot get your chin up to the bar with your traps down.
One of the Solutions
You should start releasing muscles that attach to your shoulder. Modern life is about texting, working on the computer, and driving cars with seat built for hours. The result is a forward head and rounded shoulders! Ultimately, a shoulder blade that is constantly drifting forward and up and gravity speeds up the situation, pulling you down and further out of position.
Get a lacrosse ball and start smashing. First release your pec muscles, then the teres (those little guys on the outside of your scapula that are working so hard to get you over that bar), and finally the upper trap. For the pec and teres, place the lacrosse ball on a tender spot and lean up against a wall. Lean onto the ball, and then move your arm back and forth. For the trap, you want to find a bar bell, get underneath it so your upper trap is, well, trapped underneath it, shrug up and depress your shoulder, also raise your arm up and down like a snow angel. When you release, you will know it because your body will start to melt into the lacrosse ball or bar.