I don't understand how / why the Leatt Airflex Pros kneepads are recommended so much? I bought them because of the glowing reviews here. After a few minutes into the ride, they turn into shin guards. They are light weight and comfortable but do not stay in place. I would not buy. Disappointed.
Sounds like you just got the wrong size honestly.
I have two buddies who I ride with that have them, and neither has ever complained about them sliding around/having to re-position them. I'd think if it was a problem, they wouldn't review well.
I have the airflex elbow pads, and I first bought the XL's based on the sizing chart, and then had to return/exchange them for L's, as they did slide down slightly. So maybe they just run a bit large?