DDJ 400 - 4 knobs aren't moving smooth

A few days ago, 2 of my EQ knobs started to move strangely. When i move them anti-clockwise between 12 and 11 o'clock, it feels like there is sand inbetween and the movement feels scratchy, not smooth. Now two more started to move like that. All at the exactly same position between 12 and 11 o'clock. I removed one knob to see if something is stuck in there but looked clean, so i believe the problem is within the potentiometers. How is that even possible? I never bended the knobs or turn the knobs aggressively. And more and more knobs start to move like that. Please help me