It would seem to me that because she has been a Domme, she is looking for you to be a Dom at least as strong, if not more strong, than she is herself, Clearly just giving her a command is not not enough to make her subbie to you. She appears to need someone to take control, and that will involve your physically doing so. Telling her to bend over with an implication that you will give her a spanking is not enough to prove that you are an equal Dom to her. I would suggest that you would need to actually grab her and forcefully (not playfully) place her over your knee and give her that spanking. Remembering this is a consensual relationship, and assuming that you do have a defined D/S relationship and safeword, this sort of action could not be considered non-consensual. If she behaves as you describe and truly is subbie, she wants more than commands, she wants action.

fantassy