here are some things that may be obvious, but must be said.

A) make sure you didnt put glue on the screen where the prox sensor goes, and make sure there is nothing on the sensor itself {qtip?}
B) make sure that the prox sensor is sitting snugly in its place, and that the connector is attached snugly.
C) make sure the screen you bought has a little oval window in the topright corner for the prox sensor. {by window, i mean the normal light-colored spot that will let light through, like on official screens}