How are you asking her to sidepass?
The way I train mine, a sidepass is a combination of leg, rein, and seat cues (primarily leg and rein, the seat comes on more finished horses). Because I use very similar cues for a number of actions, I make sure that I am using a very clear combination of cues for any semi-complicated maneuver.
For example, when I ask for a sidepass to the right, I pick up on the reins a bit to stop forward motion, I drop my left hip, and apply left leg. If they start moving more forward than sideways, I use a bit more rein. If they start going backward more, I let a bit of rein out. That is the exact same way I get a half-pass as well, just with different amounts of pressure on the reins and leg.
If I was riding her and she just dropped her head and continued forward when I picked up on the reins, I would likely get after her a bit. Not hard, not as punishment, just enough to let her know that when I pick up on the reins, that means that the "forward" direction is blocked and she needs to go a different way.