Slow and gentle will get you there, if she is already asking for scritches. To get Joey started, I got closer and closer while making kissy noises, letting him see my hand at all time. I'd ask him, "Can mommy get you?" in a gentle voice, then go back to the kissy noises. If he seemed receptive, I'd touch the top of his head. It took a few weeks but I went from touching the top of his head to moving my finger a couple times before he moved, to him suddenly loving every second of it, and moving his head to guide me to where he wanted the scritches. Now, I could give scritches 90 hours a day and it wouldn't be enough.
Patience is key, and always follow her lead.