It’s possible that you think you’ve been asking for feedback, but you have been doing it in a way that isn’t producing results. Maybe you occasionally mention something general to your staff like, “Hey, if anyone ever has suggestions or feedback for me, just let me know.” Or in your start-of-the-year speech to the faculty, you mention that you have an open-door policy — teachers should feel free to talk to you any time they’re having a problem.
Asking for feedback is hard, but very vital. When people have concerns, it is so important that they are able to talk about it. I've asked teachers to give me feedback in the format that we use for my actual evaluation and teacher's evaluation. There is some power in anonymity, but that also leaves room for potshots. A very healthy system is needed for that anonymous system to work. There need to be high levels of trust going both ways.
I agree with Jennifer, in that you really need to go out of your way to get feedback, regardless of what you say at any given point. You really need to go out and give people a chance and method for giving feedback.