Make sure the direction your asking her to move into is wide "open" and the off side is really "closed". Open, as in the rein is open, your leg and hip are open. Closed, if she moves into your leg, you bump her off and close that rein.
What is she doing instead of side passing? Spinning? Going forward? Going back? What you do depends on what reaction your getting from her.
Keep in mind what a horse learns on one side must also be learnt on the off side.
You need to get her front end moving and her back end moving each on their own... do you know how to do that?
If not, here you go: to get her hind end moving I want you to open your inside rein a bit and with your thumb up pull back towards your him and use your inside leg to push her hind end around. Once she crosses over let her walk out of it. Do the same going both ways. Make sure that when she does it you let her walk out of it so that she knows that is what you want. For the front end it is a little different you want to walk forward using that outside leg (not to far back though) to push her over and inside rein. Once you get each part moving then they can work together
Make sure you use something solid in front of her. A gate, a solid board fence, the side of the barn but she must have something in front of her.
Like others have said. Leave one side open, the leg totally off her, use the open side rein to move her front in that direction and your pushing leg moved back into her flank. You can move her front end over with the rein but she has to also move her rear end from the leg pressure.
I find moving her in the direction of the barn works best. She wants to go that direction so use that and move he towards the barn.
To move the other way to start you might have to choose another obsticle to put her against that moves her agian towards the barn.
Many horses side pass but the real test comes when you start side passing over obsicles.
If she backs up when you're using your cues, have her butt against a fence. It'll keep her from going backwards, and your reins can keep her from going foward, with released pressure when she moves sideways.
Remember when she's starting, even if she takes just the LITTLEST step sideways, release all your pressure and let her think about it.
When I'm first asking a horse to sidepass, I give them relief even if they only shift in that direction. That gets them thinking that that's the way to take a break. Once they get the idea and are more comfortable with it taking a proper step sideways, I'll move it up to two or four steps, but make sure you take your time with her so she doesn't get frustrated.