Why the sound trough my headphone jack is not working properly?

I have a few DS lite consoles for repair. Three of them have a problem with the sound through the headphone jack. I used the diagnose rom from https://www.gamebrew.org/wiki/Diagnose to check them fast. The speakers appear to work correctly, but when I tried with headphones trough the jack the anomaly appears. The sound test is this: when you press the left, right, up on the directional pad, the sound should come from left, right, center. The problem is this when I press left or right the sound comes from both sides with the same power at the same time. And for up(center) the sound is correct, coming from both sides at the same time, but the volume is too low. From a correctly working console you can easily recognize from which side you hear the sound and at center it doesn't lower the volume so much. As the headphone jack looks solid and I can't locate any visible damage around it, I am wondering for the cause of this problem???