Ladies, may I offer some advice? I train a couple of female athletes, and alomst all can do pullups. There are a few points, that i feel everyone should know.
Firstly, for pullups do not use the assisted dip chin machine. They are ok for training the lats, but the stabilizer muscles are greatly neglected. Eventually you may be able to ween yourself off of the machine, but it will take a while.
Next, if you do not have a spot, do negatives. Simply set a bench up underneatht the pull up bar, and use it boost yourself up after you have copleted a eccentric phase of the movement.
If you do have a spotter, use a simple progression. Begin by having the spotter hold both of your feet. After you become stronger, have your spotter just hold one leg at a time. This will help to train the stabilizing muscles of the upper back.
Well, i hope offered some advice that was beneficial to you all, and good luck with all of your goals.