I've found that performance sometimes varies based on what is in front of the receiver bouncing more IR back into the sensor. One client's setup worked perfectly until we closed the closet door. When we hung black cloth inside the closet door, the problem went away.

I've also seen ambient light not exactly interfere but seem to affect the sensitivity of the sensor. That's difficult to deal with because you can't pin that down. Maybe a big piece of black tape on the picture window, with a pinhole where a person can look out?