you are perhaps too light with your hands. I mean, you ask for something, she resists, you give up. She learned "resist the hand and it will go away".
Does this describe your situation? I mean, if you are trying to teach her to flex to the bit, you must use as much hand as it takes to get that, THEN release. Soon, she will flex very easily and you will be able to have light hands again.
However, all that is moot if she is resisting due to stiffness in her body. Since how she places her head is best as a result of how she uses her body, then one would want to focus more on that. Is she really able to move out when asked? Does she stop soft, or does she lean hard on the bit? Does she canter counter bent on a circle?
Would you be willing and interested in posting a video of her, and perhaps you riding her?
But, the short answer to your question is that if your hand is so light that the horse ignores it, then you must get stronger until you get what you ask for, then go light. The horse can have lightness if she answers it, but if you are so light that the horse just blows right through your rein cues, then stop right there, GET the thing you asked for, then go back to being light. You build in sensitivity, just as by stopping short of what you want, you build in INsensitivity.