Geoffrey:
The answer is to take the submission/symbolism out of it, however important that aspect is to you. Be patient, getting her over your knee is just the beginning. Once you have broken that barrier.....
The knee is a particular problem so is bending over. Try doing it whilst she is lying on the bed. Much more neutral. I have also employed games, card games in my case. The winner spanks the loser, nothing to do with behaviour or domination. Just be sure you can win that game (each point equals one spank) or you will end up on the receiving end.
Well, I have no problem with one-sided spanking games (I've written stories about a few), however they should be fully consensual, with the player who is quite likely to lose aware that the situation is one which probably will result in him/her receiving most or even all of the swats given on his/her posterior. If he/she wants an even chance of winning, which could involve adjusting for comparative skill levels in some games, and therefore of delivering the stinging smacks afterward, that should be the agreement.
Should a spanko-dominant husband with a 'vanilla' wife want her to engage in erotic spanking play, I'm figuring that she should have an equal opportunity to spank him, if that's what she desires as an equalizing aspect of their arrangement.
Husband: "But honey, I'm a pure spanko-top, I don't enjoy being spanked."
Wife: "Well sweetheart, I'm a total 'vanilla,' I don't enjoy it at all either."
Of course, there may be other considerations which she would accept in return for being spanked by her spouse, or she might occasionally live with having a smarting derriere simply out of marital love, even if she felt nothing sexually or otherwise personally gratifying from it.
Still, some people of both genders are going to consider the act of their being spanked to be inherently childlike and demeaning, no matter how it's presented by their potential spankers, and refuse to participate in it on that basis; many of them would undoubtedly feel the same way about administering spankings to their spouses.
This describes a cartoon from "Collected Submissions," KD Pierre's online blog:
A female teacher is spanking a young pupil, bare-bottom-up across her lap, in front of his classmates, while she tells him, "Stop whinging! You'll have to pay good money for this sort of thing when you're older." (Possibly not, assuming he'll grow up spanko-submissive, if he can find an understanding girlfriend or wife, since 'vanilla' persons tend to be much more obliging about dishing out sound bottom-warmings to their partners than undergoing them.)
"An understanding 'vanilla' lover is worth his/her weight in gold." --Classic spankophile wisdom

--C.K.