I taught both of my trail horses to sidepass. I don't show and have never had formal lessons, so I don't know how "correct" this is, but it works for me and both of my horses didn't sidepass when I bought them.
Say I want my horse to sidepass to the right, this is how I do it:
From a halt, I tilt the horse's head just slightly to the left. I also hold the right rein sort of steady, but I don't give him total slack nor really pull on that side, just some soft contact. Then I give some pressure and release with my left heel. Sometimes farther back than what I eventually want to be able to do it, if the horse doesn't seem to understand. Then I nudge with the left heel, keep the head slightly tilted to the left, and make sure my right leg is "open." I also like to give the command "over" in the hopes that they will learn what that means.
One thing I thing is important, is that you may have to make some small corrections with your hands if the horse either goes forward or backward, but don't let up with your left leg (if you are trying to go right) if the horse is going backwards or forwards or you will teach him that is the correct response (by releasing the pressure). The horse will search for the right answer, so make sure you don't accidentally reward him at the wrong time.
I kind of taught my mare (by accident) that backing up was the correct response because she would start backing when I would request a sidepass, and instead of waiting for the correct response, I would just quit nudging with my heel and start over. So I accidentally trained her to go backwards as well as sideways! We're pretty much over it now, and she sidepasses great, but sometimes when I ask for a sidepass, I will still get a back instead, and that is totally my own fault!
I guess the reason I posted this, is if you don't get success, try tipping the horse's head in the opposite direction you want to go. That seemed to help a lot. I also open my leg (by taking it off the horse's side) to encourage that as the way I want them to move. In other words, I am blocking movement in one direction and giving them an opening in the direction I want them to go.
I also practiced from the ground like you are doing as well. Even at the tie rail I would poke him/her in the ribs and say "over." Both sidepass and let me do gates out on the trail now.