It's hard to give advice without knowing more about your setup.
Well, that's not true. It's always easy to give advice; it's just
hard to give *good* advice.
How much ambient light was in the room? Were lighted dancers close
to the screen? If there was lighted stage area near the screen, you
may have been getting either bounce light (off the floor) or spill
light which was washing out your image (the same thing often happens
with lighted cycloramas, by the way). If the choreography does not
depend on the dancers' being right next to the image, put as much
space between them as you can live with.
The perceived brightness of a projected image is at least partially a
function of contrast. If there's a way to make the room generally
dimmer, your slides will seem brighter.
Jeffrey E. Salzberg, Lighting Designer