Proximity and ambient light sensor

Hi everyone, I have a problem with the proximity sensor and the ambient light sensor of my white iphone 6s. I replaced the front panel with a non-OEM part. at first the proximity sensor did not work. with an indelible marker I obscured the sensor housing and worked. if I make a call and then put the phone to my ear, the display goes off. if I receive a call and the phone is on the table, when I put it to the ear the display goes off. if instead I receive a call and take the phone out of my pocket and put it to my ear, the display will not turn off. I did a test: if I receive a call and I cover the proximity sensor before answering, the display will not turn off. if, however, I cover the ambient light sensor before answering, the display on call will turn off. if I receive a call and the phone is in my pocket, I answer, press the home button and then return to the call, the display if I put the phone to my ear, goes off. what could be the defect? thank you all