Most Ball Pythons (and snakes in general) are rather head-shy, so don't be surprised if that never goes away. I have 7 BPs myself, and only a few of them let me touch their heads... two of them were always mellow like that, but the other got used to it over time. Just be gentle and patient, always approaching with your finger from behind - not straight towards their eyes, as that is very threatening to them. You can start on the "nape" of their neck, and slowly move your finger up to the head. Like I said, this may never become something she tolerates, so don't take it personally!
P.S. I'm going to send you a PM, with a link to a great site...