Assuming that it is a stock breech sensor and stock board ->
Does the display indicate the presence of a ball (o) when an object is in the breech? Does it indicate an empty breech ( ) when there is nothing in the breech?
If not, check the sensor holes for dirt/shell; check that the sensors point into the breech; check the breech sensor wires for damage; check the connector on the board for damage or paint/dirt.
If so, check that the red sensor is on the right side of the breech and the black sensor is on the left; check that the paint does not have a clear shell which the breech sensor is seeing through.
Steve 'Flash' Monks