Couple of things…your probably not doing anything wrong. I’d double-check your video just to make sure it’s broadcast safe with nothing (or not much) above 100 ire on a waveform monitor. Projectors work off of reflected light from a screen. Often the office projectors found in conference rooms are blasting out tons of light and hitting a “high gain” screen, meaning that a lot of light is reflected back (causing your problem). This is to combat the high ambient light in the room…people want to be able to give presentations with the house lights on (for note takers, flip charts, etc…) and still be able to see the screen with the slides. Most of thes projectors are designed for text and powerpoint graphics and not video. All that being said, if you have any control over the venue and the projector before the event you might want to try to tweak the environment (turn the lights down) and spend a little time going through the projector settings to calibrate it to where it looks good. Some even have presets for video (ie. cinema mode), rather than presentations.
Professional IMAG projectors and screens you’ll find in the big venues and such will probably be calibrated better.
Erik