Hi,
You should make calibration for true discrimination. For calibration you should use tr1 trimpot. Firstly you should turn clockwise until the end. After this, you should turn counter-clockwise until see FULL on the lcd. Then you should push lcd button. If circuit can not discriminate, you should repeat to turn counter-clockwise until see FULL on the screen. Also, signal trimpot must be first quarter.
Best Regards.